In various parts of the picture and mask code, we were casting
the `clipped` rect to i32 (after rounding out). However, this
can cause overflow panics when the origin of the rect is too big.
Instead, treat the origin as f32 (which it was generally being
converted to anyway), and only cast the size part to be i32 as
required. This is safe since we know that the size has been
clipped to the visible screen, so will always be safe to cast
to i32.
Differential Revision: https://phabricator.services.mozilla.com/D80968
