Blender Git Loki

GPUShader: Add GPU_SHADER_2D_IMAGE_MULTISAMPLE_2/4/8/16

This shader is used instead of blitting back and forth to a single sample
buffer.

This means it resolves the color and depth samples and outputs a fragment
which can be depth tested and blended on top of an existing framebuffer.

We do static shader variation with manual loop unrolling for performance
reason. In my test I get 25% more perf with intel integrated gpu and 75%
performance gain with dedicated nvidia card compared to a single shader
with a uniform for sample count.