Bugfix: Revised/repaired SFML2 sample

Changed the code such that the SFML 2 sample is compatible with the
revised libraries of SFML2
Bugfix: fixed error building libRocket with MinGW

When librocket was build with MinGW the wrong platform was selected. As
a consequence the wrong code was compiled which lead to corrupted
libraries.
Revert "Bugfix: fixed error building libRocket with MinGW"

This reverts commit 66b4d56779e6c5d370298d9f1ec47f56adf8003b.
Bugfix: fixed error building libRocket with MinGW

When librocket was build with MinGW the wrong platform was selected. As
a consequence the wrong code was compiled which lead to corrupted
libraries.
Revert "Bugfix: fixed error building libRocket with MinGW"

This reverts commit 773afa3a051f9907edce387f0d66142373f098cb.
Warning fixes.
Fix gcc compilation warnings

Mostly 'converting to non-pointer type int from NULL' and vice versa
Move ElementStyleCache.* to Core
More fixes for compilation warnings
Do not precalculate kerning for all characters

Precalculating kerning for all character pairs consumed a lot of RAM (N*N complexity). For CJK fonts that contain tens thousands of glyphs it was a highly unpractical approach resulting libRocket consuming >1.5GB of RAM. Instead, encapsulate FT_Get_Kerning call in FontFaceHandle::GetKerning and let FreeType handle the kerning on the fly. In my measurements, the performance difference is negligible, especially when compiled compiled geometry is utilized.
Copy-paste fix for elements with max_length set.
added sdl2 render interface sample
Merge pull request #110 from dougbinks/master

Warning fixes.
Merge pull request #99 from RazorDE/master

Bugfix: Revised/repaired SFML2 sample
Merge pull request #111 from viciious/fixwarnings

Fix gcc compilation warnings
Merge pull request #122 from wheybags/sdl2

added sdl2 render interface sample
MINGW documentation change fromhttps://github.com/libRocket/libRocket/pull/67
Merge pull request #112 from viciious/kerning_footprint_reduce

Do not precalculate kerning
Merge pull request #1 from libRocket/master

Merge master repository in