GL_ARB_shading_language_420pack: component names sets mix is forbidden

GL_ARB_shading_language_420pack: component names sets mix is forbidden

When refering to component names we can use xywz, rgba or stpq but not
mix them when refering to the components of a specific resulting vector
or scalar.

For example, this is illegal; float(1.5).xr, but this is legal;
float(1.5).xxxx.rgba.stpq.xywz

GL_ARB_shading_language_420pack doesn't explicitly forbid it but we can
find the following in the specs:

From The OpenGL® Shading Language 4.40, page 104:

  "  v4.xgba;   // is illegal - the component names do not come from
                                the same set."

Added another case for the following test:
    GL44-CTS.shading_language_420pack.scalar_swizzlers_invalid

Change-Id: I31889598ccf88850c44eeb0e267f43f2d89bbcf2