Update CurvilinearPerspective for better depth visibility

When using the Curvilinear Assistant, I noticed that it would be better if depth was more visible in it, like the 2 point perspective. So, I changed the calculation for the assistant preview lines to create a better depth perspective instead of simply choosing points at a constant interval and added the fade effect present in the 2 point perspective assistant.

Also lowered the stroke weight, to reduce eye strain and match the 2 point perspective assistant's weight.

Before: After:

With 2 Curvilinear Perspective Assistants and a Vanishing Point Assistant (so that all 3 axes have assistants):

Test Plan

• Verify the assistant renders as shown in "After"
• Verify "Show Painting Assistants" appropriately toggles the assistant visibility.
• Verify "Show Assistant Previews" appropriately toggles the mouse preview.
• Verify toggling the assistant (with the edit assistant tool) toggles the assistant visibility.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.

Reminder: the reviewer is responsible for merging the patch, this is to ensure at the least two people can build the patch. In case a patch breaks the build, both the author and the reviewer should be contacted to fix the build. If this is not possible, the commits shall be reverted, and a notification with the reasoning and any relevant logs shall be sent to the mailing list, kimageshop@kde.org.