Render part of a document (incorrectly) on the GPU

In order to get QuadTrees working, we're going to need to
render small "parts" of a document individually. The way this works
is that, when calling RenderObjectsOnGPU, we now pass a range of
object IDs. The ObjectRenderer will then find (in O(N) time, sadly)
the range of indices needed for that object type and then render those.

Béziers, and other objects which rely on the ObjectID on the GPU being
correct might be broken by this (in some cases), though if so it shouldn't
be too hard to fix by passing an offset in as a uniform.