Add support for loading indexed image files into a texture through Texture2D.FromStream.
Fix bug where GraphicsDevice.Clear() always cleared stencil and depth buffers.

This was a consequence of moving platform-specific code that prior to
eeafc23378861d1401ebdba417431b4e850e8abf only ran for the single-parameter
form of Clear() into a PlatformClear() method used by all forms of Clear().

This change splits it out again.
Add PlayStation4 to content pipeline target platforms.
Allow full path to external tools & attempt to address some cross-platform inconsistencies.
Update README.md

Fixed links to new build server.
Update README.md

Fixed develop build artifact link.
Use single PlatformClear method and clear non-applicable options.
BasicMaterialContent now uses opaque data to all its properties.
Made texture dictionary readonly.
Use texture setter for basic material.
Added missing MaterialContent classes.
Trying to fix iOS build triggering.
Fixed malformed if test.
Disabled Video.BackgroundColor to make iOS build.
Fixed PlatformResume() for iOS.
Implement web platform (JSIL) stubs and update Protobuild
Fix vsync on WindowsGL

Affects issue #1184