Expose EXT_float_blend in Swiftshader without actually doing validation.

Also fix some missing instructions in dEQP.md.

dEQP test results: there are quite a lot failures from my locally run tests.
Not sure if they used to be there, or is introduced by my change.

16/16 of the blend tests passed.


Bug: chromium:968830
Change-Id: Ifa2bb09788899b004d62388311aa7d2aa6ad6df3
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/32888Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Sean Risser <srisser@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Shrek Shao <shrekshao@google.com>