Use the null texture as the default font + default small circle

[bsansouci]

Using the default font is super useful for drawing debugging text, but forces the global batch buffer to be flushed. If the default font had 1 px that was predictably white, we could use it as our null texture and the default font, not needing to flush for each texture.

Also small circles are needlessly expensive to draw. We could have a 20x20 pixels circle on that null texture which we'd use to draw circles with a radius smaller than 20px. It'd be very cheap to draw.

[Schmavery]

While you're at it you can close #83