Use DrawManager for Image/UV Editor

This project moves the current UV/Image editor drawing to the draw manager.
Why would we do this:

**Performance**:

Current implementation would draw each texel per time. Multiple texels could be
drawn per pixel what would overwrite the previous result. You can notice this
when working with large textures. Repeat image drawing made this visible by
drawing for a small period of time and stop drawing the rest. Now the rendering
is fast and all repeated images are drawn.

**Alpha drawing**:

Current implementation would draw directly in display space. Giving incorrect
results when displaying alpha transparent images.
This addresses {T52680}, {T74709}, {T79518}
The image editor now can show emission only colors. See {D8234} for
examples.

**Current Limitations**

Using images that are larger than supported by your GPU are resized (eg larger
than 16000x16000 are resized to 8k). This leaves some blurring artifacts. It is
a low priority to add support back of displaying individual pixels of huge
images. There is a design task {T80113} with more detail.

**Implementation overview**
Introduced an Image Engine in the draw module. this engine is responsible for
drawing the texture in the main area of the UV/Image editor. The overlay engine
has a edit_uv overlay which is responsible to draw the UV's, shadows and
overlays specifically for the UV Image editor. The background + checker pattern
is drawn by the overlay_background.

The patch will allow us to share overlays between the 3d viewport and UV/Image
editor more easily. In most cases we just need to switch the `pos` with the `u`
attribute in the vertex shader.

The project can be activated in the user preferences as experimental features.
In a later commit this will be reversed.

Reviewed By: Clément Foucault

Differential Revision: https://developer.blender.org/D8234

1 files changed, 4 insertions, 0 deletions

diff --git a/source/blender/draw/DRW_engine.h b/source/blender/draw/DRW_engine.h
index 6db3bb39643..ca5c2c94b40 100644
--- a/source/blender/draw/DRW_engine.h
+++ b/source/blender/draw/DRW_engine.h
@@ -99,6 +99,10 @@ void DRW_draw_render_loop_offscreen(struct Depsgraph *depsgraph,
                                     const bool do_color_management,
                                     struct GPUOffScreen *ofs,
                                     struct GPUViewport *viewport);
+void DRW_draw_render_loop_2d_ex(struct Depsgraph *depsgraph,
+                                struct ARegion *region,
+                                struct GPUViewport *viewport,
+                                const struct bContext *evil_C);
 void DRW_draw_select_loop(struct Depsgraph *depsgraph,
                           struct ARegion *region,
                           struct View3D *v3d,