Remove S3TC warning

[Philip Taylor]

When S3TC textures aren't supported, we decompress them automatically but show a performance warning message telling people to read the CompressedTextures page which suggests installing libtxc_dxtn (which is unmaintained and not any better than decompressing textures ourselves). This is quite common on Linux. We should simplify things for users by simply accepting the lack of S3TC silently because there's nothing useful anyone can do about it.

[philip]

(In [8981]) Fix #712 (remove S3TC warning). Add timer to S3TC decoder so it's still easy to determine if it was used.