Auto merge of #13872 - anholt:webgl-fbo, r=emilio

webgl: Add basic support for framebuffer attachments

This is by no means a complete implementation, but I've slowed down on working on it, so I think we should look at what it takes to merge the current code.  There are some major features missing, like initializing renderbuffers to 0 (uninitialized memory leak), tracking the attachments' attributes (width/height/format) for parameter requests, and lots of missing glCheckFramebufferStatus() validation.  On the other hand, this is enough to run some demos using FBOs.

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