render: Improve gradient rendering

[ChrisCPI]

Fixes #21593 .

Rather than trying to grab the record with the lowest possible ratio and drawing it, wait until the current record has been drawn up to its ratio before moving on to the next record.

Also fixes some edge cases relating to when records have the same ratio.

The tests may need some work, because:

• The Flash Player snapshots don't look correct (edges of shapes look pretty crispy), but I don't have a better way of snapshotting it.
• I'm not sure what the correct balance is for setting the tolerance/max_outliers, and I'm also not sure if having them as high as they are is acceptable (going back to the first point).

[ChrisCPI]

Thinking about it, do I also need to update visual/gradient_issue_9892's expected output to be from Flash Player? (I'll only do this myself if the change with from_gnash/misc-ming.all/GradientFillTest looks good)

[kjarosh]

Usually you'd do it in a separate PR and then rebase this one on top of that one. Otherwise you're breaking the test first and then fixing it.