epaint: Handle NaNs in Path

**Is your feature request related to a problem? Please describe.**
I'm making a performance graph and whenever there's a NaN in it (failed to measure or similar), that point on the graph prevents neighboring points from rendering properly.

**Describe the solution you'd like**
I'd like NaNs being treated as discontinuity in the path, same as end/beginning of the path.

**Additional context**
(Mesh under the line handles NaNs somewhat expectedly, you can see Path not rendering points where Mesh does)

Start by locating epaint's Path handling and compare it with the Mesh behavior shown in the issue. Reproduce a performance graph containing NaN values, then verify that NaNs create path discontinuities so neighboring points render correctly, matching the requested behavior for path beginnings and endings.

Written by the indexing model from the issue text.