reduce the size of texture for out of bound test to avoid possible failure on allocation of ArrayBuffer

It seems Chrome Canary in Windows(31.0.1604.2 (Official Build 218184) canary Aura SyzyASan) will crash on failure of allocation of ArrayBuffer when exercising the test on texture size limit. The crash issue soon disappeared and the browser worked well again for test texture size test when upgrading it to newer version 31.0.1605.1 (Official Build 218221) canary Aura.
Although I have not checked the differences in these two versions in detail, it seems It would be better to refine the test to  use a small size for testing the limit. This patch is verified using both the release/debug build from the latest chromium code and the Canary browser.