Metal back-end needs to generate 2 versions of XFB shader:
- one version without XFB emulation code for rasterized draw.
- one version with XFB emulation code for draws with rasterization
disabled.

Based on a change by le.hoang.q@gmail.com

Bug: chromium:1167763
Change-Id: I2858859e15b18701fadff09092bb08778c4a7e64
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2648636Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Le Hoang Quyen <le.hoang.q@gmail.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>