Add speed optimized rendering shader with no intensity grading for low end GPUs

[azonenberg]

The current shader does many memory writes and flops for intensity grading.

We can probably speed this up a lot, at the cost of losing intensity grading, if we just compute a min/max for each X coordinate and flood fill with alpha=1.

[azonenberg]

The intensity grading shader is now pretty fast, moving to v0.2 as this is less important.