#9 another SDL OpenGL patch

My apologies to ethermage for duplicating his work... I
had already finished most of this patch before checking
here. However, this patch addresses the issues with his
patch that wah_wah_1969 brought up (resizability with
aspect ratio, a choice for linear and nearest neighbor
filtering, switching to/from fullscreen works). Filters
are supported, and if opengl is enabled the scaling
factor is not overridden like it is in software mode.

[Anonymous]

patch adding OpenGL support to SDL version