glsl: Compile error if fs defines conflicting qualifiers for gl_FragCoord

glsl: Compile error if fs defines conflicting qualifiers for gl_FragCoord

GLSL 1.50 spec says:
   "If gl_FragCoord is redeclared in any fragment shader in a program,
    it must be redeclared in all the fragment shaders in that
    program that have a static use gl_FragCoord. All redeclarations of
    gl_FragCoord in all fragment shaders in a single program must
    have the same set of qualifiers."

This patch makes the glsl compiler to generate an error if we have a
fragment shader defined with conflicting layout qualifier declarations
for gl_FragCoord. For example:

layout(origin_upper_left, pixel_center_integer) in vec4 gl_FragCoord;
layout(pixel_center_integer) in vec4 gl_FragCoord;

void main()
{
}

V2: Some code refactoring for better readability.
    Add compiler error conditions for redeclarations like:

layout(origin_upper_left) in vec4 gl_FragCoord;
layout(origin_upper_left, pixel_center_integer) in vec4 gl_FragCoord;

and

in vec4 gl_FragCoord;
layout(origin_upper_left, pixel_center_integer) in vec4 gl_FragCoord;

V3: Simplify function is_conflicting_fragcoord_redeclaration()
V4: Check for null pointer before doing strcmp(var->name, "gl_FragCoord").

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Cc: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>