Added nvgIntersectScissor()

- added function to allow to combine scissor rects using intersection
Set commandx/y before the transform
This prevents commands like `nvgQuadTo` and `nvgArcTo` from double-transforming the previous point when they pass it back to `nvg__appendCommands`.
Merge pull request #138 from notlion/fix-double-transform

Set commandx/y before the transform