Convert image data only when necessary

Convert images only when they're not already 32bit
Swap SDL color masks for icon

So at least colors in the icon are displayed correctly
Better frame rate / time display

Calculate and update every 500ms which makes it much more readable
Improve audio and volume handling.

Prevent a deadlock in AudioManager mixer callback when stopping sounds.
Detach and free mixer handle before closing it in
VolumeControl::deinit().
Fix font rendering for big font sizes

This should fix #81. If font creation fails because we're over max.
texture size, scale the glyphs down by some amount and then upscale the
text that is rendered. This gives a somewhat blurred image, but it works
for a lot more font sizes.
Merge remote-tracking branch 'horstbaerbel/master' into unstable
Fixed a warning.
Refactored Font to use a Vertex struct and Vector2.
Fixed InputConfig error messages dumping to console and not logging.
Fixed skipped inputs being saved.