Fix bug where GraphicsDevice.Clear() always cleared stencil and depth buffers.

[jbeshir]

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.

[JamesLupiani]

Good catch, thanks! @tomspilman ?

[tomspilman]

Yes... good catch as this is a bug, but I think this is the wrong fix.

GraphicsDevice.Clear() should call GraphicsDevice.PlatformClear(ClearOptions, Vector4, float, int) like it does now. GraphicsDevice.Clear() should probably always set ClearOptions.Stencil and ClearOptions.DepthBuffer as I think those are the defaults.

Then in GraphicsDevice.PlatformClear(...) we should disable ClearOptions.Stencil and ClearOptions.DepthBuffer if stencil or depth is not bound to the current target.

@jbeshir - You can either update this branch with the correct fix and we can reopen this PR or you can submit an new PR with the right fix.

[jbeshir]

That sounds good. I'll update the branch with that fix in the next couple of days.

On OpenGL platforms I believe previously it has always tried to clear the stencil and depth buffers and only had a comment noting that later it needed to be fixed to do that filtering. Is it sufficient for this PR to simply retain that comment (perhaps with a wording update) at the top of OpenGL's PlatformClear?

I am not clear at present how to implement the missing behaviour for OpenGL, so if that needs to be bundled into the same fix that might need to wait for someone else to look at it if I can't figure it out quickly.

[tomspilman]

I'll update the branch with that fix in the next couple of days.

Great... thanks!

Is it sufficient for this PR to simply retain that comment
(perhaps with a wording update) at the top of OpenGL's PlatformClear?

Sure... you can't fix everything all the time. :)

I am not clear at present how to implement the missing
behaviour for OpenGL,

I'm not either which is why I wrote that comment in the first place. We can continue to live without it until someone finds the need to fix it.

[jbeshir]

I've added another commit to the branch which works in the suggested way; has a single PlatformClear, provides all options to it when calling it from the single parameter Clear, and removes options (or has a comment noting that it should be) that are non-applicable inside PlatformClear.

[tomspilman]

Ok... this looks good. Thanks for the contribution... merging.