Rename also classes and files for GL tests.
Use plural form also for CORRADE_UTILITY_LIBRARIES.
package: Jenkins CI configuration.
Moved development PKGBUILDs to `package/archlinux`.

They cluttered the root project dir way too much.
package: added Clang PKGBUILD.
package: enable everything possible for development PKGBUILDs.
external: updated OpenGL ES 2.0 and 3.0 headers.
OpenGL ES 2.0 and 3.0 share the same extension list.

Until recently (or maybe not too recently) ES3 extension header was
"currently empty", now the extension header is shared with ES2. It's
nice to finally get rid of all the weird ifndefs.
package: compile in Release mode for Jenkins CI.

Might point out various compiler misoptimizations.
package: Build other configurations even if the tests don't pass.

More convenient for cases when the main compiler misoptimizes something
and others do not.
Fix extension link in the documentation.
Fix ES2 method name.

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