Add texSubImage2D test to oes-texture-float test

This tests for a recent Chromium bug where the behind-the-scenes
internalformat might change if texSubImage2D replaces an entire floating
point texture, affecting renderability.
Also test for texSubImage2D changing part of a float texture
Merge pull request #477 from NVIDIA/texsubimagefloat-test

Add texSubImage2D test to oes-texture-float test