Port circle render type (v8 styles)

[kkaefer]

We need to port the point symbol type to C++. This type will be able to render plain circles with arbitrary color, which is very useful for visualizing many small features that don't require a particular icon.

There is a JS implementation already which contains the shaders and a general outline of what the feature will look like. Porting it includes:

• Parse the stylesheet into the correct structures (see style spec for how the JSON looks like)
• When parsing vector tiles into buckets, generate the OpenGL buffers required for rendering
• When rendering, add code that renders the buffers with the correct shaders
• Add tests for correct bucket creation (may not exist in JS yet)
• Add visual rendering tests (may not exist yet in the test suite)

[jfirebaugh]

⇢ #1766