glsl: Add support for representing framebuffer fetch in the GLSL IR.

glsl: Add support for representing framebuffer fetch in the GLSL IR.

The GLSL IR representation of framebuffer fetch amounts to a single
bit in the ir_variable object applicable to fragment shader outputs.
The flag indicates that the variable will be implicitly initialized to
the previous contents of the render buffer at the same fragment
coordinates and sample index.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>