This is desired for implementing WEBGL_color_buffer_float support on
WebGL1 on ANGLE's ES2 driver, for Firefox.

Bug: angleproject:3322
Change-Id: I599f86db62402333d3ef9235956c63e667f1513d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1541722Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>