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.
Merge branch 'fix-clear-options' of https://github.com/jbeshir/MonoGame into ts_scratch
Remove custom file from WindowsGL build; it is not needed for that platform.
Merge branch 'ts_scratch' of github.com:iamthefaker/MonoGame into ts_scratch