Fix pen width padding in bounding/clipping rects

[ajyoon]

The key problem was that the QClippingPath's padding was set from the pen value before the pen was set, so it was always using the default pen width even when the drawn path had a different pen. The solution was to simply defer the padding calculation to occur within update_geometry().

This change also fixes a few other related issues:

• The padding derived from pen width is now width / 2, not just the width, since Qt divides the pen stroke width evenly across the ideal path line.
• Text paths were also failing to update their clipping geometry after setting the pen.
• Paths were unnecessarily calling update_geometry twice for every render. This change adds a QClippingPath init parameter defer_geometry_calculation which allows us to improve performance by avoiding this.

Fixes #14 thanks to help from @Xavman42

[ajyoon]

@Xavman42 github won't let me assign a review to you, but if you'd like to take a look over this before I merge I'd appreciate a sanity check!

[ajyoon]

A very thick straight line:

Your example:

[Xavman42]

This looks like a big improvement to me! I wonder if this is related to the flowable clipping issues in #21. Consider this approved!

[ajyoon]

Those problems are definitely related, but I don't expect this to fix them. thanks!