Fix #116121: Resolve framebuffer resize issue in Metal

Changing size of framebuffer attachments would
throw an assertion as framebuffer size was not
correctly reset to zero. Zero allows any size
to override the current if there are no set attachments.

Authored by Apple: Michael Parkin-White

Pull Request: https://projects.blender.org/blender/blender/pulls/116162

source/blender/gpu/metal/mtl_framebuffer.mm

@@ -1242,9 +1242,8 @@ void MTLFrameBuffer::ensure_render_target_size()
   if (colour_attachment_count_ == 0 && !this->has_depth_attachment() &&
       !this->has_stencil_attachment())
   {
-
     /* Reset default size for empty framebuffer. */
-    this->default_size_set(width_, height_);
+    this->default_size_set(0, 0);
   }
 }
 
@@ -1786,7 +1785,7 @@ MTLRenderPassDescriptor *MTLFrameBuffer::bake_render_pass_descriptor(bool load_c
     }
 
     /* Attachmentless render support. */
-    int total_num_attachments = colour_attachments + (mtl_depth_attachment_.used ? 1 : 0) +
+    int total_num_attachments = colour_attachment_count_ + (mtl_depth_attachment_.used ? 1 : 0) +
                                 (mtl_stencil_attachment_.used ? 1 : 0);
     if (total_num_attachments == 0) {
       BLI_assert(width_ > 0 && height_ > 0);