Viewport Compositor: Add DEG eval callback and redraw listener

This is pretty basic but it gets the job done. This may change in
the future.

The `NodeType` (SHADING) for the `OperationKey` is not ideal.

Also it seems the tagging comming from the nodetree tag everything
as COPY_ON_WRITE update. Which is slow. To investigate.

Viewport Compositor: Add basic alphaOver node support

Not all feature are supported. For now expect both inputs to be
"premultiplied".

Viewport Compositor: Small fixes

Fix memory leak, a crash when resizing and wrong texture coordinate
in camera mode.

Change render layer sampler name to allow other texture to be bound.

Merge branch 'eevee-rewrite' into viewport-compositor

EEVEE: Film: Make smooth transition not rely on dtxl->color persistence

This is a needed change for the viewport compositor. The compositor
needs to draw to `dtxl->color` to have correct overlay / background
composition.

The solution here is to have a separate buffer that keeps the first
sample we blend from. This increases VRAM usage but it is the most
elegant option.

Viewport Compositor: Initial commit

This introduce a new compositor engine. It applies the compositor
nodetree onto the render result in the viewport using GLSL shader.

For now it only very few nodes are supported and only the combined
pass is passed to the evaluation pass.

This reuse almost the same pipeline as `GPUMaterial`.

DRW: Add barebone realtime compositor engine

This is just the initial commit. Compositor just clears the framebuffer to
red.

EEVEE: Use special depth shader to resolve the depth buffer

This was the cause of a bug on Intel Integrated GPU and might as well
impact other platforms.

EEVEE: LightGrid: Fix divide by zero leading to wrong world lighting

This integer divide by zero was evaluated to 0 on all platform but apple,
where it yields 1. The world lighting would then sample the 1 sample of the
first grid instead of its own sample.

EEVEE: Film: Fix undefined viewport color values after resizing

This was caused by the blend mode that was used even with full opacity.
This caused issues with the viewport was resized and the color of the
framebuffer becomes undefined, leading to undefined values in the blend
equation.

Another fix would be to clear the viewport color on resize inside the
GPUViewport.