Implements filters

[amylizzle]

Fixes #294 by implementing filter() and supporting cast.

• [x] Implement /dm_filter type
• [x] Implement basic shader processing in UpdateAppearance()
• [x] Get basic debug shader working (greyscale)
• [x] Finish the DreamFilter set parameter code
• [x] Make DreamViewOverlay use RenderInRenderTarget
• [x] Sort out garbage collection for shader instances
• [x] Implement multiple shader support

Working shaders:

• [x] outline
• [x] greyscale (this one isn't actually in DM, it was just a debug thing, but I could see it being used)
• [x] Gaussian blur

Things this PR does

• Fairly heavily modifies the current rendering system to a "ping-pong" rendering system, which allows for repeated applications of shaders where needed.
• Adds the filter() built-in proc, which returns a DreamObject of type /dm_filter
• Adds handling for the .filters var on atoms, with appropriate appearance update and listifying behaviour (as per BYOND)
• Adds the DreamFilterMetaObject which adds appropriate appearance update behaviour to /dm_filter objects
• Adds the DreamFilter type, which is basically just a meta-data object that can be passed over the network in IconAppearance and holds the necessary data to instantiate the requested shader on the client
• Adds some shader caching & garbage collection behaviour to ClientAppearanceSystem so we can keep a handle on shaders which share parameters, and clean up shader instances which aren't in use anymore.
• Adds a "Filter Demo" button to the TestGame so you can pick from a list of filters with default settings and see that it works.

##Examples BYOND left, OD right Blur:

Outline:

[Altoids1]

This PR is ridiculous and I am so happy someone else handled this nonsense for us, thanks a bunch

[amylizzle]

~~TODO: change this to /dm_filter, the ENTIRELY UNDOCUMENTED type path of filters in DM~~

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