ANGLE_iosurface_client_buffer imports IOSufraces with GL_RGBA +
GL_HALF_FLOAT which isn't a valid GLES internalFormat + type
combination so sampling from this format fails.  We cannot easily
update the spec on this format because it is also implemented by
SwiftShader so manually adjust the format to sized RGBA16F.

Bug: chromium:1148599
Change-Id: I3f24bbdcd08003eaae26e8391e5edca7a51f6888
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2558950Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Geoff Lang <geofflang@chromium.org>