Added nvgCancelFrame()

This commit implements a new nvgCancelFrame() function for the purpose described in #168.

Currently, if calling nvgBeginFrame(), the nvgEndFrame() function must be called to finish the drawing. However, if knowing the drawing would take too much time and want to give up the drawing. There is no way to cancel it. nvgCancelFrame() gives another choice to end the frame without actually drawing.
Merge pull request #169 from olliwang/cancel

Added nvgCancelFrame()
Fixed syntax error of checking defined identifier

The syntax is wrong when checking defined identifier.
Merge pull request #170 from olliwang/static

Fixed syntax error of checking defined identifier
Consistent code syntax and typo fix
Merge pull request #174 from SmilyOrg/patch-1

Consistent code syntax and typo fix
Merge latest changes from memononen/nanovg