Make textTenderingMsdf work in compat mode

[greggman]

dpdxFine and dpdyFine are not available in compat mode. switched them to dpdx and dpdy.

• textRenderingMsdf is broken #555

[greggman]

setting them to dpdxCoarse dpdyCoarse on NVidia Windows (D3D) I don't see any artifacts. I scaled transform for the text.

[toji]

I was just about to upload the same thing. :) Glad you're in a position to test it on Windows, though. I was going to do that later today.

For what it's worth the AA actually has a lot more impact (and potential artifacts) at a smaller scale. See screenshot here for an example. I'd actually be curious if there's any loss in readability if you shrink the text a bit? Assuming it's good, though, this LGTM.

[greggman]

here's the sample's current size

Here's 1/2 size

Here's that zoomed in 4x

Here's 1/2 size with dpd(x/y)Fine

[toji]

That doesn't strike me as noticeably worse. 👍 Thanks for doing the comparison and merging!