gallium: add support for GLES texture float extensions (v3)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=74329



v2: add a CAP for half floats
    drivers should not expose the CAPs if they don't support the formats

v3: update relnotes

Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>