glsl: Link error if fs defines conflicting qualifiers for gl_FragCoord

glsl: Link 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 causes the shader link to fail if we have multiple fragment
shaders with conflicting layout qualifiers for gl_FragCoord.

V2: Restructure the code and add conditions to correctly handle the
    following case:

fragment shader 1:
layout(origin_upper_left) in vec4 gl_FragCoord;
void main()
{
    foo();
    gl_FragColor = gl_FragData;
}

fragment shader 2:
layout(pixel_center_integer) in vec4 gl_FragCoord;
void foo()
{
}

V3:
Allow linking in the following case:
fragment shader 1:
void main()
{
    foo();
    gl_FragColor = gl_FragCoord;
}

fragment shader 2:
in vec4 gl_FragCoord;
void foo()
{
   ...
}

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