This PR adds a clear on creation for draw region framebuffers, which
fixes issue https://projects.blender.org/blender/blender/issues/139443.
When using the "Lock Interface" option, redrawing is disabled until the
rendering process has finished. When the GPU driver does not initialize
framebuffer textures with zero, this means that visual corruption can
be perceived when resizing the main window while rendering.
This fix makes sure that framebuffer textures are initialized with zero
on all GPU platforms.
Pull Request: https://projects.blender.org/blender/blender/pulls/139445
