Expand conformance/textures/misc/gl-teximage.html with more alpha=0 cases

conformance/textures/misc/gl-teximage.html already contains a test uploading a PNG with a zero alpha channel into an RGB texture, and ensuring the alpha channel was not premultiplied into the color channels.

Per comments on #2451 , this test should be expanded to verify uploading into RGBA, RGBA_4444, and RGBA_5551 textures, and ensure that the alpha channel is in fact zero for all of these formats (as well as the colors being preserved as well as possible for the texture format).

Start in conformance/textures/misc/gl-teximage.html, where the existing PNG alpha=0 RGB-texture test is defined. Extend the test coverage to RGBA, RGBA_4444, and RGBA_5551 uploads, verifying zero alpha and preservation of color values as well as each format allows; done means all requested cases are checked.

Written by the indexing model from the issue text.