WebPositive SVG stroke rendering

[thebuck]

The handcrafted stroke-test.svg uncovers many stroke-related rendering issues, see Conclusions section.

Using the test image:

• View it standalone in WebPositive to test mouse hover effects and to avoid #17305.
  
• Every assymmetry (except color change on mouse hover) with respect to the vertical red line is a browser bug.
  

Elements of the image (left | right):

• orange top: filled rectangular path | stroked line path
  
• blue top U: just one curve path
  
• black O: differently scaled quarter paths with their stroke-width and geometry adjusted to make them look identical;
  listing stroke-width:
  
  • top: 4 | 0.125
    
  • bottom: 16 | 2.5
    
• blue bottom: line path segments | one dashed line path
  
• While the mouse is over an element, the element becomes green.
  

Conclusions from my hrev55788/x86_64/QEMU test run:

1. If 0 < stroke-width < 1 then it renders as 1.
2. transform is neglected for path polygonization detail level.
3. stroke-dasharray is unimplemented.
4. The combination "fill:none;stroke:#000" causes rendering to use the fill from previous colored fill element for over-stroking???
5. Bounding box of paths neglects stroke at the start- and endpoints.

What tests led to conclusions:

1. The magenta zero-stroke thing is invisible, the O has thick top right (0.125 as 1), and bottom right was not rounded to int (2.5).
2. The number of polylines (or corners, look carefully) in O quarters increases with stroke-width, i.e. with less upscaling
3. The bottom blue right half is continuous, also for mouse hover detection.
4. Scrolling the bounding box of the top left orange rectangle in/out of view toggles orange overpainting of all black elements (resize window for this).
5. Hovering the mouse over elements works only in their bounding box and repaints only the same (test especially blue elements; move between them and the red cross to get larger repaints).