[CP-Beta] Fix Rect::ExpandToMinTransformedSize to return the input rectangle when no expansion is needed, and remove 1-pixel roundrect to rect simplification - #189885

…when no expansion is needed, and remove 1-pixel roundrect to rect simplification (flutter#189808)

Fixes flutter#189807

See the issue for more context.

Adds a unit test that fails without this change, and passes with this
change.

Verified the demo app in the issue renders properly with this change.

From the issue:

## Root cause and fix


[Rect::ExpandToMinTransformedSize](https://github.com/flutter/flutter/blob/1c8b3088238220102df66761685f3a4718b5aa15/engine/src/flutter/impeller/geometry/rect.h#L705)
always constructs a new expanded rectangle from the input Rect from its
center point and half-extents.

If the input rectangle is already large enough and does not need
expansion, this should return an identical rectangle. The returned
rectangle is equality-checked against the input in
[Canvas::DrawRoundRect](https://github.com/flutter/flutter/blob/1c8b3088238220102df66761685f3a4718b5aa15/engine/src/flutter/impeller/display_list/canvas.cc#L1129)
to determine whether expansion happened. That code assumes that if the
output does not equal the input, pixel-expansion happened, which means
that the rounded rect had sub-pixel dimensions and can be safely drawn
as a non-rounded rectangle.

The issue is that for certain cases, particularly with non-integer
sizes, ExpandToMinTransformedSize returns a different rectangle even if
no expansion happened. This is due to small floating point imprecisions
when constructing the new rectangle using the input rectangle's center
and half-extents.

To fix this, ExpandToMinTransformedSize should not construct a new
rectangle unless expansion is actually needed.

## Removal of 1-pixel roundrect to rect simplification

This PR also removes the 1-pixel roundrect to rect simplification in
`Canvas::DrawRoundRect`. As described in
flutter#189807 (comment),
this optimization resulted in slightly different colors at the tips of
thin roundrects.

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we expect every widget to implement].
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- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
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Fixes] where supported.
- [ ] All existing and new tests are passing.

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