[Win] Check registry keys before accessing

Fixes crashes when using OpenTK over the Remote Desktop Client for Mac
(version 2010).
[Win] Fix issue #35 (OpenTK over Remote Desktop)

When running over remote desktop without hardware acceleration, there
are no GraphicsModes that support desktop composition. This patch adds
logic to avoid requesting composition-capable modes when running over
RDP.

Additionally, it changes the mode selection logic to consider modes that
support features partially (e.g. 16bpp color instead of 32bpp), albeit
with a heavy penalty over fully supported modes.
Merge branch 'rdp' into develop