This change adds a new composite mode called `CompositeModeMultiply`,
which multiplies the source color with the destination color.
This is tested on Linux and Windows only.
Fixes#410
This change is an experimental implementation to use Pixel Buffer
Objects. This reduces calls of glTexSubImage2D.
This works only on desktops. Unfortunately WebGL does not have
this features. Mobiles can have PBO as of OpenGL ES 3.
Updates #976