Fix #112586: VSE can't use multi-layer EXR images

The rest of blender does handle multi-layer EXR images, using the
"combined" or RGBA/RGB layers when the visual result is needed. Make
VSE do the same.

While fixing this, I found several issues in other not well tested code
and had to fix them:

- IMB_buffer_float_from_float_threaded was wrongly using source channels
  as destination channels, producing garbage result.
- IMB_scale_into_new was not assigning channels to destination image.

Pull Request: https://projects.blender.org/blender/blender/pulls/132790

source/blender/imbuf/intern/divers.cc

@@ -518,7 +518,7 @@ static void imb_buffer_float_from_float_thread_do(void *data_v, int scanline)
   const int num_scanlines = 1;
   FloatToFloatThreadData *data = (FloatToFloatThreadData *)data_v;
   size_t offset_from = size_t(scanline) * data->stride_from * data->channels_from;
-  size_t offset_to = size_t(scanline) * data->stride_to * data->channels_from;
+  size_t offset_to = size_t(scanline) * data->stride_to * 4;
   IMB_buffer_float_from_float(data->rect_to + offset_to,
                               data->rect_from + offset_from,
                               data->channels_from,

source/blender/imbuf/intern/scaling.cc

@@ -879,6 +879,7 @@ ImBuf *IMB_scale_into_new(
 
   /* Create result image. */
   ImBuf *dst = IMB_allocImBuf(newx, newy, ibuf->planes, IB_uninitialized_pixels);
+  dst->channels = ibuf->channels;
   IMB_metadata_copy(dst, ibuf);
   dst->colormanage_flag = ibuf->colormanage_flag;
   if (dst_byte != nullptr) {

source/blender/sequencer/intern/render.cc

@@ -843,6 +843,30 @@ static ImBuf *seq_render_effect_strip_impl(const SeqRenderData *context,
 /** \name Individual strip rendering functions
  * \{ */
 
+static void convert_multilayer_ibuf(ImBuf *ibuf)
+{
+  /* Load the combined/RGB layer, if this is a multi-layer image. */
+  BKE_movieclip_convert_multilayer_ibuf(ibuf);
+
+  /* Combined layer might be non-4 channels, however the rest
+   * of sequencer assumes RGBA everywhere. Convert to 4 channel if needed. */
+  if (ibuf->float_buffer.data != nullptr && ibuf->channels != 4) {
+    float *dst = static_cast<float *>(MEM_mallocN(sizeof(float[4]) * ibuf->x * ibuf->y, __func__));
+    IMB_buffer_float_from_float_threaded(dst,
+                                         ibuf->float_buffer.data,
+                                         ibuf->channels,
+                                         IB_PROFILE_LINEAR_RGB,
+                                         IB_PROFILE_LINEAR_RGB,
+                                         false,
+                                         ibuf->x,
+                                         ibuf->y,
+                                         ibuf->x,
+                                         ibuf->x);
+    IMB_assign_float_buffer(ibuf, dst, IB_TAKE_OWNERSHIP);
+    ibuf->channels = 4;
+  }
+}
+
 /**
  * Render individual view for multi-view or single (default view) for mono-view.
  */
@@ -855,7 +879,7 @@ static ImBuf *seq_render_image_strip_view(const SeqRenderData *context,
 {
   ImBuf *ibuf = nullptr;
 
-  int flag = IB_rect | IB_metadata;
+  int flag = IB_rect | IB_metadata | IB_multilayer;
   if (strip->alpha_mode == SEQ_ALPHA_PREMUL) {
     flag |= IB_alphamode_premul;
   }
@@ -873,6 +897,7 @@ static ImBuf *seq_render_image_strip_view(const SeqRenderData *context,
   if (ibuf == nullptr) {
     return nullptr;
   }
+  convert_multilayer_ibuf(ibuf);
 
   /* We don't need both (speed reasons)! */
   if (ibuf->float_buffer.data != nullptr && ibuf->byte_buffer.data != nullptr) {

source/blender/sequencer/intern/strip_add.cc

@@ -211,8 +211,9 @@ void SEQ_add_image_init_alpha_mode(Strip *strip)
 
     /* Initialize input color space. */
     if (strip->type == STRIP_TYPE_IMAGE) {
-      ibuf = IMB_loadiffname(
-          filepath, IB_test | IB_alphamode_detect, strip->data->colorspace_settings.name);
+      ibuf = IMB_loadiffname(filepath,
+                             IB_test | IB_multilayer | IB_alphamode_detect,
+                             strip->data->colorspace_settings.name);
 
       /* Byte images are default to straight alpha, however sequencer
        * works in premul space, so mark strip to be premultiplied first.
@@ -255,7 +256,8 @@ Strip *SEQ_add_image_strip(Main *bmain, Scene *scene, ListBase *seqbase, SeqLoad
   char file_path[FILE_MAX];
   STRNCPY(file_path, load_data->path);
   BLI_path_abs(file_path, BKE_main_blendfile_path(bmain));
-  ImBuf *ibuf = IMB_loadiffname(file_path, IB_rect, strip->data->colorspace_settings.name);
+  ImBuf *ibuf = IMB_loadiffname(
+      file_path, IB_rect | IB_multilayer, strip->data->colorspace_settings.name);
   if (ibuf != nullptr) {
     /* Set image resolution. Assume that all images in sequence are same size. This fields are only
      * informative. */