cameraEffects: make sRGB conversion work in older GLSL

[illwieckz]

I don't know which GLSL version allowed mix() to received both bvec3 and vec3, maybe I can just use the second writing everytime, I don't know if using bvec3 as input brings some optimizations or not.

[slipher]

LGTM

[illwieckz]

With MESA_GL_VERSION_OVERRIDE and MESA_GLSL_VERSION OVERRIDE, without this branch I get the error with GL 2.1 and GLSL 120 but not with GL 3.0 and GLSL 130, so I assume that version test is correct.

[illwieckz]

For reference the compilation error message was:

Warn: Compile log:
0:108(10): error: no matching function for call to `mix(vec3, vec3, bvec3)'; candidates are:
0:108(10): error:    float mix(float, float, float)
0:108(10): error:    vec2 mix(vec2, vec2, float)
0:108(10): error:    vec3 mix(vec3, vec3, float)
0:108(10): error:    vec4 mix(vec4, vec4, float)
0:108(10): error:    vec2 mix(vec2, vec2, vec2)
0:108(10): error:    vec3 mix(vec3, vec3, vec3)
0:108(10): error:    vec4 mix(vec4, vec4, vec4)