Fix tex-new-formats to reflect WebGL2 sRGB semantics

I didn't realize this when I wrote this test, but this is a substantial difference between WebGL2 (i.e. ES 3) and the old EXT_sRGB. In WebGL2 / ES3, there is not even a SRGB_ALPHA constant. Instead, people are expected to use RGB and RGBA for format. In other words, WebGL2 / ES3 does not have 1:1 mapping between sized formats, and (unsized format, type) pairs. The (RGBA, UNSIGNED_BYTE) pair corresponds to both plain old RGBA8, and to SRGB8_ALPHA8.