Implement accessing textures through an index. It uses the same
interface as OpenGL, the main difference is that Vulkan bindings are
forced to be arrayed (the binding index doesn't change for stacked
textures in SPIR-V).
Some games like The Legend of Zelda: Breath of the Wild assign
render targets without writing them from the fragment shader. This
generates Vulkan validation errors, so silence these I previously
introduced a commit to set "vec4(0, 0, 0, 1)" for these attachments. The
problem is that this is not what games expect. This commit reverts that
change.
