Workaround segfault in SDL when switching to opengl.

Clearing the screen after setting videomode with opengl segfaulted. I didn't
check in detail, but looks like a problem in SDL and re-initializing the video
subsystem did the trick.
fix scrollTo/drag on scrollbars
Merge pull request #830 from cpages/glfix

Workaround segfault in SDL when switching to opengl.
Added audio format options.
Fixed audio subsystem not being properly reinitialized.
Fixed old resources staying in memory after reload.