Added GetDeviceCaps (windows platform)

We will use this method to read out the logical DPI of the monitor and
scale window elements accordingly.
Improved WinGLContext initialization

Cleaned up and added debugging information to the temporary context
construction in WinGLContext. Simplified WinGraphicsMode constructor.
Corrected core profile selection

According to GLX_create_context, the correct flag for a core profile is
GLX_CONTEXT_CORE_PROFILE_BIT_ARB.
Cleaned up context profile selection

Added support for WGL_create_context profiles and added methods for the
selection of context flags and profile.