This patch adds the texture pool functionality that was previously
only available in the DRW module to the GPU module.
This allows to not rely on global `DST` variable for the managment
of these temporary textures.
Moreover, this can be extended using dedicated GPU backend
specific behavior to reduce the amount of memory needed
to render.
The implementation is mostly copy pasted from the draw implementation
but with more documentation. Also it is simplified since the
`DRW_texture_pool_query` functionality is not needed.
Pull Request: https://projects.blender.org/blender/blender/pulls/134403
