glsl: Define a gl_LastFragData built-in for older GLSL versions.

glsl: Define a gl_LastFragData built-in for older GLSL versions.

The EXT_shader_framebuffer_fetch extension defines alternative
language for GLES2 shaders where user-defined fragment outputs are not
allowed.  Instead of using inout user-defined fragment outputs the
shader is expected to read from the gl_LastFragData built-in array.
In addition this allows using the same language on desktop GLSL
versions prior to 4.2 that support the deprecated gl_FragData built-in
in preparation for the MESA_shader_framebuffer_fetch desktop GL
extension.

Both legacy and user-defined inout outputs have a common
representation at the GLSL IR level, so it shouldn't make any
difference for optimization passes and back-ends whether the
application is using gl_LastFragData or user-defined outputs, all
they'll see is a variable dereference of a fragment output at a
certain interface location with the fb_fetch_output bit set to one.

v2: Don't define the built-in variable on GLSL versions for which
    gl_FragData exists but is deprecated. (Ken)

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