Although according to
https://docs.microsoft.com/en-us/windows/desktop/direct3ddxgi/format-support-for-direct3d-feature-level-9-1-hardware
D3D9 doesn't have full blending capability for RGBA32F. But turns out it could provide
correct blending result in reality. As a result of some regression reports by client app, we
decided to turn floatBlend on for D3D9.

I tried to make a native D3D9 program to detect float blending capability.
But turns out it needs quite some effort on Win10 for a D3D newbie.
Considering the fact that D3D9 backend is not fully passing conformance test anyway.
I don't think it worth the effort. Seems to be okay to just turn it on.

Bug: chromium:970532
Change-Id: I66bec120be6eaaa62a5472b8521a97b4d23e924a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1652731Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Shrek Shao <shrekshao@google.com>