Fixed Mouse Button State Issues

Buttons would cancel each others ButtonState.Pressed  out, as well
rotating the mousewheel would cancel all ButtonState.Pressed
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.
Merge pull request #2322 from jbeshir/support-indexed-images

Add support for loading indexed image files into a texture through Texture2D.FromStream
Merge pull request #2343 from SickheadGames/PS4PlatformEnum

Add PlayStation4 to content pipeline target platforms.
Use single PlatformClear method and clear non-applicable options.
Merge pull request #2335 from jbeshir/fix-clear-options

Fix bug where GraphicsDevice.Clear() always cleared stencil and depth buffers.
BasicMaterialContent now uses opaque data to all its properties.
Made texture dictionary readonly.
Use texture setter for basic material.
Added missing MaterialContent classes.
Merge pull request #2355 from SickheadGames/matfix

MaterialContent Fixes
Trying to fix iOS build triggering.
Fixed malformed if test.
Disabled Video.BackgroundColor to make iOS build.
Fixed PlatformResume() for iOS.