Define MAGNUM_TARGET_GLES3 if targetting ES and not ES2.

Makes it easier to disable parts of the code than with this. And this
would also not be future-proof:

    defined(MAGNUM_TARGET_GLES) && !defined(MAGNUM_TARGET_GLES2)
Added FindOpenGLES3.cmake module.

Seems that Mesa has both ES2 and ES3 in the same libGLESv2.so. Weird.
16bit SNORM internal texture formats are not available in ES3.

Don't know why I thought they were.
Some image & texture formats are available only through ES2 extensions.

And ES3 has no extensions now.
Texture border & anisotropy is available only as ES2 extension.
No deprecated Buffer::map() in ES3.
glDrawBuffer() is not available in ES3.

Using glDrawBuffers() also in this case.
Platform: forgot to initialize variable.
Updated PKGBUILDs and added ES2/ES3 variants.