mesa: Add new MESA_FORMATs for ETC2 compressed textures

mesa: Add new MESA_FORMATs for ETC2 compressed textures

It is required by OpenGL ES 3.0 to support ETC2 textures.
This patch adds new MESA_FORMATs for following etc2 texture
formats:
 GL_COMPRESSED_RGB8_ETC2
 GL_COMPRESSED_SRGB8_ETC2
 GL_COMPRESSED_RGBA8_ETC2_EAC
 GL_COMPRESSED_SRGB8_ALPHA8_ETC2_EAC
 GL_COMPRESSED_R11_EAC
 GL_COMPRESSED_RG11_EAC
 GL_COMPRESSED_SIGNED_R11_EAC
 GL_COMPRESSED_SIGNED_RG11_EAC
 MESA_FORMAT_ETC2_RGB8_PUNCHTHROUGH_ALPHA1
 MESA_FORMAT_ETC2_SRGB8_PUNCHTHROUGH_ALPHA1

Above formats are currently available in only gles 3.0.

v2: Add entries in texfetch_funcs[] array.

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
v3 (Paul Berry <stereotype441@gmail.com>): comment out symbols that
are not implemented yet, so that this commit compiles on its own;
future commits will uncomment the symbols as they become available.