QPdf: Force miter limit into valid range

Miter limits below 1.0 are not valid in PDF and trigger an error in the
Reader. This change enforces a minimum miter limit of 1.0, and it adds
a manual test for producing a PDF file demonstrating this issue.

Task-number: QTBUG-52641
Change-Id: I1c78b1c4a44579e95e1cddfb459926d304e60165
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>