glsl: Handle the inout qualifier in fragment shader output declarations.

glsl: Handle the inout qualifier in fragment shader output declarations.

According to the EXT_shader_framebuffer_fetch extension the inout
qualifier can be used on ESSL 3.0+ shaders to declare a special kind
of fragment output that gets implicitly initialized with the previous
framebuffer contents at the current fragment coordinates.  In addition
we allow using the same language to define FB fetch outputs in GLSL
1.3+ shaders in preparation for the desktop MESA_shader_framebuffer_fetch
extensions.

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