shaders: revise sh_lut method logic

This is required to support GLSL ES 1.0 and GLSL 110, which forbid the
use of literal arrays in shaders. Since SH_LUT_LITERAL is now no longer
a safe fallback, we instead always fallback to SH_LUT_UNIFORM.

This is technically an API break, since in the past, the naked pl_shader
API would always generate literal shaders, but now they may have arrays
attached as uniforms - to prevent this, users can still set small LUT
sizes (which is what e.g. VLC does anyway)