core: Cull text under textfield

[adrian17]

This should optimize cases where there's a lot of text left to be scrolled under the textfield. The speedup can be extreme if there are hundreds of pointlessly-drawn lines.

(However, note that this doesn't help with the opposite case - when you scroll to the bottom, it'll still draw all the text :/ )

The perfect solution might be to implement all 4 types of culling (lines under/above* textfield/screen), but with all the transforms, height metrics etc etc this was the only one I could confidently implement - and even in this one I'm not convinced I didn't skip any edge cases.

(Note that due to how scrollV works, lines above the textfield could be skipped without even iterating over them. In fact... maybe FP (and us) could deduce in-bounds for each glyph, thus making us not require masking at all?)

[adrian17]

Here's a demonstration.
This was recorded with text masking disabled, so text outside the textfield is visible - except lines culled by this PR.
Right textfield is normal, left textfield has scrollV = 3.

ruffle_desktop_2024-03-14_19-19-17.mp4