Clean up quick rejection, especially surrounding points + lines.

bug:4351353

quickReject and quickRejectNoScissor have been renamed and refactored:
- to make the scissor side effect clear and explicit
- dangerous methods no longer public
- to make the simple quick reject check logic const
- simple quick reject is now conservative

This CL also fixes several issues with line and point quickRejection -
sub-pixel and hairline lines are much less likely to be incorrectly
rejected, especially at small canvas scale.

Additionally, alpha modulation for AA points < 1px in size is now
correct, dumplicating SW behavior (similar to lines and stroked
shapes work).

Change-Id: Ibb0710c721b9fb415d05acf54dd3d2b4d602156a