Provide scale offset to WindowVertex calculation

Summary:
WindowVertex contains the x position and also the relative texture x position

Our textures are scaled bigger than kwin's resolution, so this makes sure we set the right
texture position.

Otherwise our final array ends up using 0 to 0.5 of the texture, not 0 to 1.

Test Plan:
Opened some 2x windows, on a 1x output.
It looked how it should do, instead of it being double the size and cropped

Tested some things that use quads, like wobbly windows

Reviewers: #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D3502