Added checks to build script to detect MacOS and to
build Mac and iOS if the correct tools are installed
Fixed a few issues with the MacOS detection
Add CONTRIBUTING document.
Fixed a problem with the IsDisposed Implementation in Gamer.cs
Merge remote-tracking branch 'upstream/develop' into gamerfix
Merge pull request #1439 from dellis1972/gamerfix

Fixed issue with Gamer IsDisposed implememtation
Add check for GL_EXT_texture_filter_anisotropic before texture anisotropy used
Code review action - fix incorrect logic and adding missing GL error check
Merge pull request #1442 from Aranda/no_tex_aniso

Add check for GL_EXT_texture_filter_anisotropic before using anisotropic filtering
Merge pull request #1438 from slygamer/CONTRIBUTING

Add CONTRIBUTING document.