On low-DPI screens, we don't promote fixed- or sticky-position
elements unless we have to for other reasons. The reason is to preserve
LCD text.

However, this higher-quality text rendering comes at a big cost:
- main-thread scrolling
- confusing and corner-casey invalidation
- performance cliffs for developers

Let's remove this feature, now that a significant fraction of our
userbase doesn't even encounter this situation.

Note that LCD text will still happen for fixed- and sticly-position elements as long as they have integral positions, and have simple-
to-detect opaque backgrounds.