Fix scissoring of VBO rendering in multi-screen setup on Wayland

Summary:
On Wayland in multi-screen a "window" is used to render to per screen.
In OpenGL every "window" is at 0/0 and scissoring is in respect to the
window geometry. The rendering geometry passed to VBO rendering, which
performs the scissoring, is in virtual combined screen geometry space.
Thus the scissoring always removes any rendering on the second screen.
This results in e.g. wobbly windows not working on any additional screen
on Wayland.

This change makes the VBO rendering multi-screen aware by passing the
current virtual window geometry in each rendering pass. That replaces
the so far used virtual screen size. For single screen rendering or
multi-screen rendering on X11 this doesn't change anything as there is
always only one OpenGL window and the screen geoemetry starts at 0/0.
Thus the subtracting of x/y geometry do not affect the scissoring at
all.

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D2955