SVG: read a NaN or infinite gradient coordinate as 0, like an invalid one - #2118

A gradient coordinate that can not be parsed, such as x1="bad", is read as 0 with a warning. But float() accepts nan and inf, so <linearGradient x1="nan"> wrote /Coords [nan 0 1 0] into the PDF, and <radialGradient r="nan"> a NaN radius, neither a valid PDF number.

A NaN or infinite coordinate (also as a percentage, -inf%) is now handled like any other invalid one: a warning, and 0. For r, that means the existing "invalid radius, skipping" path.

Test added: the gradient renders exactly like one with a coordinate of 0, for both gradient types (the NaN and infinite cases fail on master).

• A unit test is covering the code added / modified by this PR
• A mention of the change is present in CHANGELOG.md
• This PR is ready to be merged