Fix broken path in WEBGL_compressed_texture_etc1
Fix incorrect numbers in extensions

In pull request 337 (commit cb547d713104a) I incorrectly updated
the extension numbers for three community reviewed extensions. Revert
this.

Also, add a status date for the compressed texture ETC1 extension.
Assigned extension #24 to WEBGL_compressed_texture_etc1 extension.
Merge pull request #432 from kenrussell/etc-number

Assigned extension #24 to WEBGL_compressed_texture_etc1 extension.
Added history entry for assignment of number to WEBGL_compressed_texture_etc1.
Merge pull request #433 from kenrussell/etc-history

Added history entry for assignment of number to WEBGL_compressed_texture...