Remove flat varying

[djg]

This addresses Issue #3004

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This change is 

[jrmuizel]

thread 'validate_shaders' panicked at 'Shader compilation failed: cs_border_segment
ERROR: 0:599: 'mod' : no matching overloaded function found
ERROR: 0:599: '/' : wrong operand types - no operation '/' exists that takes a left-hand operand of type 'in highp float' and a right operand of type 'const int' (or there is no acceptable conversion)
', webrender/tests/angle_shader_validation.rs:143:13

[djg]

thread 'validate_shaders' panicked at 'Shader compilation failed: cs_border_segment
ERROR: 0:599: 'mod' : no matching overloaded function found
ERROR: 0:599: '/' : wrong operand types - no operation '/' exists that takes a left-hand operand of type 'in highp float' and a right operand of type 'const int' (or there is no acceptable conversion)
', webrender/tests/angle_shader_validation.rs:143:13

Because 2 is not the same as 2.0 🤦‍♂️