Measuring took the current font and scale, so UIContext switched to the
style being measured and back. On first draw the font to switch back to
could be an empty one, which the UWP drawer asserts on.
MeasureString and MeasureStringRect now take the style and scale, and the
backends' MeasureStringInternal measures in the style it's given.
SetOrCreateFont, which sets the font drawing uses, is shared, and the
backends only implement EnsureFont, which creates a font if needed.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
