Transparent viewmodels

[nooodles-ahh]

This PR primarily implements transparent viewmodels in the same vein as the mod found in TF2, originally described here.
To briefly explain the technical implementation; rather than changing the render amount, which causes you to see obscured model geometry and doesn't look very good, the viewmodel is rendered to a separate texture and drawn over the frame buffer. The texture is created by writing to the stencil buffer, then clearing the alpha channel of the texture according to that.
A gross part of the implementation is the repurposing of the STUDIO_DRAWTRANSLUCENTSUBMODELS flag to control the model draw and drawn functions. This was done so I can stencil out sprites drawn with the viewmodel as it looks a lot nicer than the sprite just showing through and stenciling doesn't really deal with transparency. The naming kind makes sense but I'm happy to redefine the flag with a different name or alter the implementation a bit.
This also implements IClientRenderTargets for HL2 derivative mods so I also added hl2_water_resolution and hl2_monitor_resolution convars copied over from TF2's implementation.

Screenshots

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