Filter: blur() support

[garrynewman]

2ab7204f-b76b-49be-878c-e4097e4df959.mp4

I had to rewrite some stuff to support filter: blur( 10px ), so I'm gonna make a couple of lists in case people run into trouble with it. Let me know in the replies if I fucked you.

Known Issues:

• Children of panels with a filter style don't support overflow propertly
• Children of panels with a filter style probably can't have filters themselves - let me know if this is a big issue and I'll put the work in
• Children of panels with a filter style will get weirdness with backdrop-filter - again let me know if this is an issue
• Alpha intensity seems different

Changes:

• Added support for filter: blur( xx )
• Render.Clear becomes Render.ClearAttributes
• Added Render.Clear( color, cleardepth )
• Added Render.SetRenderTarget( texture )
• Added Render.RestoreRenderTarget() (restores RT to backbuffer, maybe should be SetRenderTarget( null )?)
• Texture.CreateRenderTarget() now returns a TextureBuilder instead of a TextureRenderTargetBuilder
• TextureRenderTargetBuilder deleted
• Added TextureBuilder.WithSize( size )
• Added TextureBuilder.WithMultiSample2X() (and others)
• Added TextureBuilder.WithFormat( format )
• Added TextureBuilder.WithScreenFormat()
• TextureBuilder.Create throws exception if size is <= 0
• Labels explicitly delete their textures when they're deleted
• BaseTextBlock is now internal
• UI blend modes write alpha (important when drawing to a render texture)