opengl: sampler: fix swizzle components
The GLSL allows three different swizzling:
- .rgba
- .stuv
- .xyzw
However, they are not allowed to be mixed together:
ERROR: 0:8: Inconsistent component characters in swizzle 'xa'
ERROR: 0:10: Use of undeclared identifier 'pixel'
ERROR: 0:11: Use of undeclared identifier 'result'
Fix the swizzling to w instead of a.