Now that these formats are emulated if necessary on ES 3.0 contexts,
advertise filtering support as well. This enables
OES_texture_half_float_linear on mobile devices which support ES 3.0
but don't advertise this extension.

Bug: angleproject:4648
Change-Id: I09f00fbb1454b2cfa72295254fbc45c872f4919c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2210701Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>