Vulkan distinguishes between a texture format that can be used as a
render target (VK_FORMAT_FEATURE_COLOR_ATTACHMENT_BIT) and one that
supports blending (VK_FORMAT_FEATURE_COLOR_ATTACHMENT_BLEND_BIT).
This cl adds a blendable capability and sets it based on the
appropriate bit. It also adds the check in GetFormatSupportBase(),
but all uses of it are currently set to false.

In terms of behavior, this cl should be noop.

Bug: angleproject:4291
Change-Id: Ib151ac8ab7b69311b7fdef15ad87186d13e3ed78
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1999494
Commit-Queue: Alexis Hétu <sugoi@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>