mesa: add entry AMD_shader_trinary_minmax to extension_table

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glsl: Add extension tracking for AMD_shader_trinary_minmax
glsl: add a shader_trinary_minmax predicate
glsl: Implement min3 built-in function
glsl: implement max3 built-in function
glsl: implement mid3 built-in function
mesa: enable AMD_shader_trinary_minmax
glsl: switch AMD_shader_trinary_minmax support flag to dummy_true
docs/relnotes/10.1.html: report AMD_shader_trinary_minmax support