[platforms/drm] Fix cursor hotspot for inverted landscape rotation

Summary:
This change fixes the hotspot calculation by introducing a dedicated
matrix to map the hotspot.

To better test the hotspot a small test application is added. The app
renders a red point at the cursor position and uses a cross for cursor
with the hotspot position being transparent. So if KWin renders the
cursor at the correct position a red point is visible in the center of
the cross. The test app supports several cursor hotspot positions:
 * center (default)
 * top left
 * top right
 * bottom right
 * bottom left

To switch between the variants press the space key.

Test Plan:
Rotated screen, used test app to verify the hotspot is
correct. Also scaled display, still correct

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D8668