Make the generator generate calls for the correct glBindAttribLocation()
calls.
Usually this was done correctly, but we can't rely on it. So do it
explicitly.
A variation is a #define/specialization constant that every shader can
use to specialize itself as it sees fit.
This commit adds the infrastrcture, future commits will add
implementations.
There's multiple uses I want it for:
1. Generating the box-shadow area for blurring
2. Generating masks for rounded-rect masking
3. Optimizing the common use case of rounded-clip + color
Only the last one is implemented in this commit.
