glsl: Add extension tracking for AMD_shader_trinary_minmax
glsl: add a shader_trinary_minmax predicate
glsl: add min() and max() functions to builder.cpp
glsl: Implement min3 built-in function
glsl: implement max3 built-in function
glsl: implement mid3 built-in function
mesa: enable AMD_shader_trinary_minmax
docs/relnotes/10.1.html: report AMD_shader_trinary_minmax support