glsl: do not emit error for non written varyings on OpenGL ES
glsl: do not emit error for non written varyings on OpenGL ES

Patch fixes following test case from 'shaders-with-varyings' WebGL
conformance suite: "vertex shader with unused varying and fragment
shader with used varying must succeed"

v2: emit still a warning if the condition happens (Ian)

Signed-off-by: Tapani Pälli <cdcb67a5e49297de94340ca09f25994bd5e7f762@intel.com>
Reviewed-by: Ian Romanick <2b237cafb16dc45038e85df6c85e74e6d899eba9@intel.com>

tokyovigilante/glsl-optimizer,mcanthony/glsl-optimizer,jbarczak/glsl-optimizer,bkaradzic/glsl-optimizer,jbarczak/glsl-optimizer,metora/MesaGLSLCompiler,zeux/glsl-optimizer,benaadams/glsl-optimizer,benaadams/glsl-optimizer,mcanthony/glsl-optimizer,djreep81/glsl-optimizer,djreep81/glsl-optimizer,mcanthony/glsl-optimizer,dellis1972/glsl-optimizer,zz85/glsl-optimizer,zz85/glsl-optimizer,dellis1972/glsl-optimizer,mcanthony/glsl-optimizer,wolf96/glsl-optimizer,bkaradzic/glsl-optimizer,jbarczak/glsl-optimizer,djreep81/glsl-optimizer,tokyovigilante/glsl-optimizer,dellis1972/glsl-optimizer,zz85/glsl-optimizer,wolf96/glsl-optimizer,bkaradzic/glsl-optimizer,mcanthony/glsl-optimizer,benaadams/glsl-optimizer,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,wolf96/glsl-optimizer,benaadams/glsl-optimizer,zz85/glsl-optimizer,jbarczak/glsl-optimizer,tokyovigilante/glsl-optimizer,tokyovigilante/glsl-optimizer,tokyovigilante/glsl-optimizer,dellis1972/glsl-optimizer,zeux/glsl-optimizer,wolf96/glsl-optimizer,zeux/glsl-optimizer,jbarczak/glsl-optimizer,zz85/glsl-optimizer,zeux/glsl-optimizer,bkaradzic/glsl-optimizer,zz85/glsl-optimizer,zeux/glsl-optimizer,bkaradzic/glsl-optimizer,benaadams/glsl-optimizer,wolf96/glsl-optimizer,djreep81/glsl-optimizer,djreep81/glsl-optimizer,dellis1972/glsl-optimizer,benaadams/glsl-optimizer