Bug fix: label widget not passing alpha channel.
Print error source with OpenGL errors.
Use stb_image for loading images.
Added downscale_image method from SOIL. Buggy at the moment.
Some down-scaling problems fixed.

The down-scaling code from SOIL expecting resizing only by the powers
of two, so when an image is to be shrunk, its width and height are
divided by two again and again until both are in the acceptable range.
Set loaded texture wrap parameter to clamp.

This fixes the bleeding effect that was sometimes seen.
Removed some unnecessary code.