Bug 1082486 Part 5 - Hide touch caret when PresShell is suppressing painting. r=roc

When loading an html, painting is suppressed in PresShell. Therefore the
rect of nsCaret or the rect of nearest ancestor scroll frames will not
be correct. Touch caret will hide incorrectly because it cannot get the
necessary rect to calculate its position.

I added a condition in IsDisplayable() to skip when painting is
suppressed. Touch caret should sync its visibility with nsCaret again
when painting is unsuppressed, and those rects needed by touch caret
should be ready then.