[Impeller] dont use half precision constants / Fixes for SPIRV tools roll

[jonahwilliams]

See b/335381180

In an upcoming version of SPIRV tools, these constants are being flagged as invalid. Due to some combination of the macros + inlining we're ending up with multiple precision modifiers on the constants:

third_party/flutter_engine/impeller/entity/shaders/radial_gradient_fill.frag: GLSL to SPIRV failed; Compilation error. 0 error(s) and 1 warning(s).
third_party/flutter_engine/impeller/entity/shaders/radial_gradient_fill.frag:14: warning: '' : all default precisions are highp; use precision statements to quiet warning, e.g.:
third_party/flutter_engine/impeller/entity/shaders/radial_gradient_fill.frag:          "precision mediump int; precision highp float;" 
shaderc: internal error: compilation succeeded but failed to optimize: ID '112' decorated with RelaxedPrecision multiple times is not allowed.
  %float_n0_474999994 = OpConstant %float -0.474999994

We don't really benefit from half precision on these, since we're either using them for equality or can cheaply convert to half precision anyway.

[flutter-dashboard]

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[jiahaog]

Even with these changes, the duplicate constant error is still showing up:

shaderc: internal error: compilation succeeded but failed to optimize: ID '27' decorated with RelaxedPrecision multiple times is not allowed.
  %float_0 = OpConstant %float 0

Also, similar changes are needed to:

engine/impeller/entity/shaders/filters/yuv_to_rgb_filter.frag

Lines 16 to 17 in 132d364

	const float16_t kBT601LimitedRange = 0.0hf;
	const float16_t kBT601FullRange = 1.0hf;

[jonahwilliams]

Updated. Sorry, do you mean that even with this change there are still compilation errors?

[jiahaog]

Yeah that's what I meant. The compiler doesn't like 0 and 1 as constants for this file and yuv_to_rgb_filter.frag.

[jiahaog]

The latest commit 80579dd fixes the issue 🙂

[jonahwilliams]

woot, thanks for helping with this @jiahaog