Allow to use glBlendFunc() for custom blending.

Originally there are only NVG_STRAIGHT_ALPHA and NVG_PREMULTIPLIED_ALPHA for alpha blend. This would cause any custom glBlendFunc() calls to be ignored. This commit adds NVG_CUSTOM_ALPHA to make glBlendFunc() calls working.

Signed-off-by: Olli Wang <olliwang@ollix.com>