GPUViewport is creating a bunch of framebuffer textures for itself, but
some space types never initialize/use them. E.g. Sequencer, Nodes etc.
only ever use the "overlay" texture. Eventually when viewport is
"drawn", it combines this uninitialized texture data and then only by
luck it happens that most of the time it is black. But not always!
The textures were only cleared (right now) on Metal backend, under
GPU_clear_viewport_workaround as if it was some driver workaround. Stop
doing that, and just clear them always.
However, there was seemingly a performance issue on OpenGL, when this
clear was being done. At least on my machine (Win10, Geforce RTX
3080Ti), the overhead of doing the clears is measurable, and is caused
by usage of GL4.4 glClearTexImage instead of a framebuffer clear. As if
glClearTexImage makes "pixel data to exist" on the CPU side and then
later on binding this framebuffer sends off that data to the GPU, or
somesuch.
More details in the PR.
Pull Request: https://projects.blender.org/blender/blender/pulls/131518
074df4ceeb