Fix excessive re-creation of VR viewport textures

Due to the freeing and re-creation of textures performed when binding
offscreen viewports, VR viewport textures would be needlessly
re-created every drawing iteration, leading to a negative impact on VR
frame rate.

This was brought to light by 6738ecb64e, which introduced an
additional texture clear operation on initialization and was
prohibitively costly on some systems when performed every frame.

Now, the textures for VR viewports will not be always re-created
during offscreen binding, but only when necessary using a pre-drawing
step (`wm_xr_session_surface_offscreen_ensure()`).

Reviewed By: jbakker, fclem

Differential Revision: https://developer.blender.org/D14059

source/blender/draw/intern/draw_manager.c

@@ -1765,6 +1765,8 @@ void DRW_draw_render_loop_offscreen(struct Depsgraph *depsgraph,
                                     GPUOffScreen *ofs,
                                     GPUViewport *viewport)
 {
+  const bool is_xr_surface = ((v3d->flag & V3D_XR_SESSION_SURFACE) != 0);
+
   /* Create temporary viewport if needed or update the existing viewport. */
   GPUViewport *render_viewport = viewport;
   if (viewport == NULL) {
@@ -1774,7 +1776,7 @@ void DRW_draw_render_loop_offscreen(struct Depsgraph *depsgraph,
     drw_notify_view_update_offscreen(depsgraph, engine_type, region, v3d, render_viewport);
   }
 
-  GPU_viewport_bind_from_offscreen(render_viewport, ofs);
+  GPU_viewport_bind_from_offscreen(render_viewport, ofs, is_xr_surface);
 
   /* Just here to avoid an assert but shouldn't be required in practice. */
   GPU_framebuffer_restore();

source/blender/gpu/GPU_viewport.h

@@ -79,7 +79,9 @@ void GPU_viewport_colorspace_set(GPUViewport *viewport,
 /**
  * Should be called from DRW after DRW_opengl_context_enable.
  */
-void GPU_viewport_bind_from_offscreen(GPUViewport *viewport, struct GPUOffScreen *ofs);
+void GPU_viewport_bind_from_offscreen(GPUViewport *viewport,
+                                      struct GPUOffScreen *ofs,
+                                      bool is_xr_surface);
 /**
  * Clear vars assigned from offscreen, so we don't free data owned by `GPUOffScreen`.
  */

source/blender/gpu/intern/gpu_viewport.c

@@ -199,7 +199,9 @@ void GPU_viewport_bind(GPUViewport *viewport, int view, const rcti *rect)
   viewport->active_view = view;
 }
 
-void GPU_viewport_bind_from_offscreen(GPUViewport *viewport, struct GPUOffScreen *ofs)
+void GPU_viewport_bind_from_offscreen(GPUViewport *viewport,
+                                      struct GPUOffScreen *ofs,
+                                      bool is_xr_surface)
 {
   GPUTexture *color, *depth;
   GPUFrameBuffer *fb;
@@ -208,7 +210,13 @@ void GPU_viewport_bind_from_offscreen(GPUViewport *viewport, struct GPUOffScreen
 
   GPU_offscreen_viewport_data_get(ofs, &fb, &color, &depth);
 
-  gpu_viewport_textures_free(viewport);
+  /* XR surfaces will already check for texture size changes and free if necessary (see
+   * #wm_xr_session_surface_offscreen_ensure()), so don't free here as it has a significant
+   * performance impact (leads to texture re-creation in #gpu_viewport_textures_create() every VR
+   * drawing iteration).*/
+  if (!is_xr_surface) {
+    gpu_viewport_textures_free(viewport);
+  }
 
   /* This is the only texture we can share. */
   viewport->depth_tx = depth;