Merge pull request #1273 from SickheadGames/FontProcessor

DXT Compression implemented
Errors now go to stderr.
Fixed missing #if WINDOWS
Added built-in System and Math content type writers.
Reader type is now correctly derived from writer type.
Merge branch 'develop3d' of https://github.com/mono/MonoGame into pipeline
Merge pull request #1275 from SickheadGames/mgcb2

Small MGCB Fixes
Merge branch 'develop3d' of https://github.com/mono/MonoGame into pipeline
MathHelper - added comments

added comments and added method overload of Clamp
fix new method in mathHelper
Merge pull request #1276 from slygamer/pipeline

Add built-in System and Math content type writers
Merge pull request #1277 from Shqrdx/develop3d

Test commit for MonoGameTeam. Added comments to MathHelper.
Added some tests to check for SDK's before building
Fixed issue with build script
removed call to installer as that will be a seperate task
First run PVRTC Support for the pipeline.
Changed function name in PVRTexLibC. Added configuration for x64.
Added PVRTCBitmapContent.