Added a test to render data using different strides *and* pointer values.

This test renders two colored triangles. The vertex data is stored in two different VBOs. The vertex data contains the same types of values (2 floats for vertex, 4 bytes for color) but uses different stride and pointer values.

The test creates two VBOs with empty data, and then for each buffer:
1. Binds to the buffer
2. fills it with `glBufferSubData`
3. calls `glVertexPointer`
4. calls `glColorPointer`
5. calls `glDrawArrays`

Each time through, the parameters for `stride` and `pointer` change when calling `gl*Pointer` while the other parameters stay the same. The cached renderer in the emscripten code should handle this just fine. I've verified the correct result using SDL on Mac OS X. The included image shows the correct result.