Render to heatmap framebuffer in separate pass

[jfirebaugh]

Heatmap rendering currently happens entirely in the translucent pass, and first binds a texture-backed FBO, draws to it, then rebinds the default framebuffer. Framebuffer binding is relatively expensive, and the second rebinding could be avoided by drawing to the texture-backed FBO in a separate draw pass, similar to the passes used by fill extrusions and hillshading. @mollymerp suggests generalizing these three into a general "framebuffer" pass.