second attempt at avoiding warnings at /W4 on vs2013
use -1 as the 'no scissor' value, rather than 0, so that 0 width still scissors
clamp on scissor, fix nvgReset
Merge pull request #121 from sgraham/zero-w-scissor

use -1 as the 'no scissor' value, rather than 0, so that 0 width still scissors
Merge pull request #120 from sgraham/warnings-attempt-2

second attempt at avoiding warnings at /W4 on vs2013
Font stash: store pref glyph as index instead of pointer