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.
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.