GLSL: Handle more intrinsics, fix bugs in code generation and texture transfer

Map intrinsics to correct GLSL functions:
- trigonometric functions (sin, cos, tan, asin, acos, atan, atan2)
- exponential functions (pow, exp, log)
- general functions (floor, ceil, lerp)

Since GLSL doesn't support bit arithmetic, don't use C backend to generate
code for "Mod" and "Div".
Why not embed in int32 instead? (I'm assuming there's a reason, but it's not obvious to me at the moment)