Add `resolution` parameter to `InkyMockImpression.__init__`

[corranwebster]

This permits simpler setting of the size of the Inky Impression being mocked.

For example, this allows:

display = InkyMockImpression((800, 480))

which will ensure that the buffer size gets set up correctly.

Fixes #168.

[Gadgetoid]

Oof, may need a rebase now I've merged #167

[Gadgetoid]

I have added this as a commit onto my WIP repackaged branch here: https://github.com/pimoroni/inky/pull/181/commits/fe70476f43d0867ee3c53190d3d2ed39c4ac12b7