Bug 882766 - Recognize clang even when it doesn't #define __GNUC__; r=ted

Clang on Windows #defines _MSC_VER but not __GNUC__, so we need to make sure
that the build system can recognize it even when __GNUC__ is not defined.
Bug 752034 - Use the Windows SDK when building with Clang on Windows; r=ted
Bug 878032 - Part 1: Factor out some simle none-RenderTarget dependent functions. r=jrmuizel
Bug 878032 - Part 2: Factor out partial bitmap uploading and scaling. r=jrmuizel