[Impeller] Add subpass blend goldens

[bdero]

Adds a checked macro "loop" for the blend modes in the style of FOR_EACH_DISPLAY_LIST_OP + a quick golden for each blend mode that runs the EntityPass subpass blending paths.

We should also add Aiks tests to verify the filters in a more comprehensive way (including the blend filter). I can do this in a follow-up. The BlendFilter actually uses different shaders than SaveLayer for the advanced blends on Metal -- the two code paths are wildly different, and we've had serious regressions for both cases.

Screen.Recording.2023-04-03.at.1.48.40.AM.mov

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Changes reported for pull request #40879 at sha 4a3fe49

[gaaclarke]

The code looks good to me modulo a couple cleanup comments.

Do you think these will be easy to parse these results by human reviewers? The documentation I saw for the blend modes uses pngs of parrots. Do you think maybe we should just copy the documentation's example?

Here's some documentation from skia that could be used that is a bit more simple too: https://skia.org/docs/user/api/skblendmode_overview/

The idea being that it would be easy to look at the documentation to see the difference between what it should be and what is being rendered.

[gaaclarke]

Add this to an anonymous namespace to avoid collisions.

[bdero]

Done.

[gaaclarke]

I'd turn i into a parameter so that the macro isn't making assumptions about the context it's used in.

[bdero]

Hoping to keep this param out of the macro input of IMPELLER_FOR_EACH_BLEND_MODE if that's cool, since it shouldn't be needed for actual usage. Open to alternative ideas for verifying the macro matches the enum.

[gaaclarke]

It's not a huge deal but here's how you could do it without macros that implicitly capture their context. We don't actually care about the asserted value, just that they are all accounted for, they all exist and there is no duplicates:

#define _IMPELLER_ASSERT_BLEND_MODE(blend_mode) \
  auto enum_##blend_mode = static_cast<std::underlying_type_t<BlendMode>>( \
      BlendMode::k##blend_mode);                                           \
  (void)enum_##blend_mode;
  
#define _IMPELLER_PLUS_ONE(blend_mode) + 1

static constexpr inline bool ValidateBlendModes() {
  IMPELLER_FOR_EACH_BLEND_MODE(_IMPELLER_ASSERT_BLEND_MODE);
  int tally = 0  IMPELLER_FOR_EACH_BLEND_MODE(_IMPELLER_PLUS_ONE);
  if (tally - 1 !=
      static_cast<std::underlying_type_t<BlendMode>>(BlendMode::kLast)) {
    return false;
  }
  return true;
}

[bdero]

This is close, but it doesn't quite prevent duplicates. If kSrcOver was accidentally replaced with a second kSrc (thereby dropping kSrcOver), I think this check would pass?

[gaaclarke]

nope, you'd get a redefinition error when compiling, enum_Src would be declared twice.

[bdero]

ah, nice

[bdero]

Do you think these will be easy to parse these results by human reviewers?

Yeah I think so. Assuming what's being rendered is currently correct, someone introducing a regression will be able to compare with the previous correct golden.

[bdero]

I do like the idea of using the w3c duck. ;) Maybe I can follow-up with that?

Also made a quick issue about improving our blend docs to add inline examples: flutter/flutter#124048

[gaaclarke]

lgtm I think we could make the tests easier to parse since looking the goldens I can't easily tell if the are correct. This will at least make sure we don't change from the current logic accidentally.

[gaaclarke]

It's not a huge deal but here's how you could do it without macros that implicitly capture their context. We don't actually care about the asserted value, just that they are all accounted for, they all exist and there is no duplicates:

#define _IMPELLER_ASSERT_BLEND_MODE(blend_mode) \
  auto enum_##blend_mode = static_cast<std::underlying_type_t<BlendMode>>( \
      BlendMode::k##blend_mode);                                           \
  (void)enum_##blend_mode;
  
#define _IMPELLER_PLUS_ONE(blend_mode) + 1

static constexpr inline bool ValidateBlendModes() {
  IMPELLER_FOR_EACH_BLEND_MODE(_IMPELLER_ASSERT_BLEND_MODE);
  int tally = 0  IMPELLER_FOR_EACH_BLEND_MODE(_IMPELLER_PLUS_ONE);
  if (tally - 1 !=
      static_cast<std::underlying_type_t<BlendMode>>(BlendMode::kLast)) {
    return false;
  }
  return true;
}