Previously, we were expecting triangles and quads to consist of
complete sets of vertices. However, a more common behavior is to ignore
all vertices that do not make up a full primitive. For example, OpenGL
specifies for `GL_QUADS`:
"The total number of vertices between Begin and End is 4n + k, where
0 ≤ k ≤ 3; if k is not zero, the final k vertices are ignored."
This changes the behavior of `Device::draw_primitives()` to both return
early if no full set of vertices was provided, and to ignore any
additional vertices that are not part of a full set.
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