As this package does not require anything except the shared library libGLEW.so.1.10 (that is to say, it does not require headers, etc.), it should RDEPEND on ~media-libs/glew-1.10.0 (or, semi-equivalently, on "|| ( media-libs/glew:1.10 media-libs/glew:0/1.10 )"), not just on the sub-slot :0/1.10. This will allow media-libs/glew:1.10 to provide the dependency without preventing other packages from using newer versions of media-libs/glew.
https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=ef780faca8479f0b2a930785cc2b02f561995d05