glsl: never lower precision for shaders in desktop GL contexts
this causes all kinds of conflicts for drivers that support 16bit ops, as mediump will be converted to 16bit in shaders that expect 32bit, which breaks various assumptions in the shaders
cc: mesa-stable
fixes a number of tests for zink: GTF-GL46.gtf21.GL.atan.atan_vec2_frag* GTF-GL46.gtf21.GL.mat3.mat3arrayindirect0_frag