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.
Merge branch 'master' into fairPsionics