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
- it keeps kernel sizes somewhat resolution independent so it will look
just as blurry across multiple screens.
::doCachedBlur still needs doing.