Shader performance improvements

[mmozeiko]

This improves performance for rect and blur shaders.

For UI rect shader - main change is moving color & sdf_sample_pos calculation to vertex shader, and then letting hardware rasterizer to do interpolatation of them instead of manual arithmetic in shader (the fewer varyings the better!)

And blur shader now does bilinear sampling to read texture samples - hw bilinear lerp reads two pixels in on sample operation. The weights are adjusted to calculate the same blurring weights.

Plus some minor math simplifications in vertex shaders.