Add radial bar.

Uses single large texture with fixed bar width.
Next step: Allow variable bar width and radius.
More general radial bar (debug).

Test: In the level, you should see a radial health bar. Hold b to make it
decrease.
Implement general radial progress bar.

graphics/hud/RadialBar.cs : Specify an origin, radius, bar thickness, bar color to
create.
During draw, pass in the current and max values.
Remove unneeded texture