add nvgSetTransform

I have tried to make a animate fill, So I should apply matrix to fill, and restore matrix. But `nvgSave`/`nvgRestore` can not use, because fill itself is the subject of `nvgSave`/`nvgRestore`, so I had to read matrix before, and after `nvgFillPaint`, restore the matrix. but NanoVG lack the `nvgSetTransform` to restore matrix.