[Impeller] Add Rect::GetNormalizingTransform to handle UV coordinate conversion

[flar]

Three places in the code were manually computing the UV coordinates relative to a texture coverage rectangle while also transforming the points. This change will make it easier both to compute the UV conversion matrix and also to consolidate it with the transform that was already being applied to streamline the total computations.

[flar]

Unit tests are included for the new method which should prevent the test-bot from complaining, but reviewers should also assess whether the 4 places that use the new method are already adequately covered by existing rendering tests.

@bdero @jonahwilliams

[jonahwilliams]

LGTM

[flar]

Unit tests are included for the new method which should prevent the test-bot from complaining, but reviewers should also assess whether the 4 places that use the new method are already adequately covered by existing rendering tests.

To answer my own question, I instrumented the 4 cases that use the new code and all but the first case in fill_path_geometry.cc is being executed in the golden tests:

% git grep "Using case"
impeller/entity/geometry/fill_path_geometry.cc:    FML_LOG(ERROR) << "Using case 1";
impeller/entity/geometry/fill_path_geometry.cc:        FML_LOG(ERROR) << "Using case 2";
impeller/entity/geometry/geometry.cc:  FML_LOG(ERROR) << "Using case 3";
impeller/entity/geometry/vertices_geometry.cc:    FML_LOG(ERROR) << "Using case 4";

% out/host_debug_unopt_arm64/impeller_golden_tests --working_dir=./imp_golden 2>&1 | grep "Using case" | sort | uniq -c
  10 [ERROR:flutter/impeller/entity/geometry/fill_path_geometry.cc(124)] Using case 2
  14 [ERROR:flutter/impeller/entity/geometry/geometry.cc(82)] Using case 3
   2 [ERROR:flutter/impeller/entity/geometry/vertices_geometry.cc(235)] Using case 4

@jonahwilliams double-checking your review still stands?

[flutter-dashboard]

Golden file changes have been found for this pull request. Click here to view and triage (e.g. because this is an intentional change).

If you are still iterating on this change and are not ready to resolve the images on the Flutter Gold dashboard, consider marking this PR as a draft pull request above. You will still be able to view image results on the dashboard, commenting will be silenced, and the check will not try to resolve itself until marked ready for review.

Changes reported for pull request #47775 at sha 23e60b7

[flar]

The golden failures were due to getting the order of the matrix multiplies backwards which only caused a problem when the effect_transform was not Identity. In diagnosing that I discovered that we had no tests that covered cases 1 and 4 with an effect transform so I updated the aiks playground tests to make sure those paths were tested with an effect transform. The coverage when I last ran the tests looks complete in terms of the test cases hit:

% out/host_debug_unopt_arm64/impeller_golden_tests --working_dir=./imp_golden 2>&1 | grep "Using case" | sort | uniq -c
  10 [ERROR:flutter/impeller/entity/geometry/fill_path_geometry.cc(124)] Using case 2
  10 [ERROR:flutter/impeller/entity/geometry/fill_path_geometry.cc(96)] Using case 1
  14 [ERROR:flutter/impeller/entity/geometry/geometry.cc(82)] Using case 3
   4 [ERROR:flutter/impeller/entity/geometry/vertices_geometry.cc(235)] Using case 4

[flutter-dashboard]

Golden file changes are available for triage from new commit, Click here to view.

Changes reported for pull request #47775 at sha 4e1604f

[jonahwilliams]

Still LGTM