Summary:
This patch caputres from the framebuffer using the framebuffer's
geometry, factoring in scale. We then keep the current normal DPI
framebuffer causing it to downsample there.

This is good because:
- it keeps the code very simple
- it's a performance optimisation. Blurring on 4k is naturally more
expensive than at regular DPI. Downsampling keeps it the same - and you
can't see a difference given it's high DPI and you're going to blur it
anwyay.
- it keeps kernel sizes somewhat resolution independent so it will look
just as blurry across multiple screens.

::doCachedBlur still needs doing.

Test Plan:
Ran an app
Ran the kwindowsystem blur test
Observed the right part of the window being blurred

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D4963