Add a test ensuring bufferData can handle different sizes

The buffer-sizes test will allocate buffers of different sizes ranging
from 3 kB to 4 TB, and ensure that either 1) the buffer is allocated with
the correct size and it is cleared before usage 2) the implementation
reports a gl.OUT_OF_MEMORY error or 3) the context is lost. Buffers <= 48
MB are expected to work on all implementations.

The test is placed in the extra folder because of the risk of losing the
context, which will cause problems in some current browsers if it happens
in the middle of conformance tests.
Merge pull request #387 from NVIDIA/extras-bufferdata-test

Add a test ensuring bufferData can handle different sizes