LinearGradient/RadialGradient: raise a clear error for bounds outside [0, 1] or decreasing - #2134

LinearGradient(..., bounds=[1.5, 0.8]) wrote /Bounds [1.5 0.8] into the stitching function, and [-0.2, ...], decreasing values or NaN went through the same way. The PDF specification requires the bounds to be increasing and within the function's domain [0, 1], and the docstring already says they are "in the range (0, 1)".

The gradient now raises ValueError: Bounds must be increasing numbers between 0 and 1: ..., next to the existing check on the number of bounds. Equal bounds and bounds of exactly 0 or 1 are still accepted, so the gradients built from SVG and shape_*_gradient() stops are unchanged.

• 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