If dpr initially unknown, update in subsequent page impression

[seb303]

Fix to address the issue mentioned here:
#3

Have submitted a similar PR for the Matomo Core Resolution plugin.
matomo-org/matomo#22080

[johsin18]

Hi seb303,
I support your PR. However, could you please make it more complete

• adapt FAQ
• add a test
• add a changelog entry

[seb303]

Hi johnsin18,
I've updated the FAQ. I don't feel confident to add a test though as I'm not familiar with the testing framework used. I'm more likely to break something than to make the code safer! Sorry.

[johsin18]

Hi seb303,
I'm on it, have to get back into the whole topic. Please have a bit more patience.
johsin18