Change clear colour for Intel performance.

THIS BREAKS XNA COMPATIBILITY!

Modern (Sandybridge +) Intel Integrated Graphics chipsets have
a fast clear/blit unit ('blorp') which, at least under Mesa,
can only do fast color clears if all of the channels are being cleared
to either full (1.0 or 255) or empty (0). The default XNA discard
colour, CornflowerBlue, does not have this property. This patch
changes the discard colour to black (0,0,0,255) as this is the colour
likely to be best supported by hardware.

This patch gets rid of a per-clear warning on Intel Ivybride/Mesa 10.0 git
using ARB_debug_output.
Don't change DiscardColor on DEBUG, fix comment
Merge pull request #2124 from sulix/clear-colour-intel

Change clear colour for Intel performance.
Merge branch 'develop' of git://github.com/mono/MonoGame into monogame-sdl2