b=810252 remove default-unused Pango shaping code r=jfkthame

--HG--
extra : transplant_source : %19%97%F6%D5%F7h%1D-Yg%D6%08%226h%BE%D3%E4%12%22
b=810274 ensure scrolled range is non-negative even in the presence of nscoord overflow r=roc

Instead of trying to detect all integer overflow at the source, this merely
keeps the sizes non-negative.  That may not mean they are less than
nscoord_MAX.

If we give in and accept that integer arithmetic may result in the scrolled
rect being smaller than the scroll port, then we shouldn't assert.

--HG--
extra : transplant_source : RP%15%84zx%B9%E8%2C%1E%0A%AB%82%F4%9C%A2y%AD5%27
b=810274 include bounds with layer pixels as possible destinations to minimize subpixel scrolls r=roc

--HG--
extra : transplant_source : %22c%20R%E6%BF%C6%10%02%7C%1E%9F%CA%C7%2B%8C%98A%28%D3
b=810274 allow smooth scroll step to nearest pixel boundary short of desired position r=roc

--HG--
extra : transplant_source : %CF%FE%015%5E%E6R%F0W%EB%1EoW%A5%3B%04%D9%F4C%0A
partially re-expand assertion count range for "non-root reflow roots must not have scrollable overflow" assertion that doesn't always trigger b=810274

range reduced in 9419b209de19