scenes/opengl: Calculate item transforms differently

Currently, a vertex coordinate is transformed first, then mapped to
the global screen coordinates. This causes a problem if a transform
is applied to the top-most item and child items are not at (0, 0).
For example, scaled windows may have popping out sub-surfaces, etc.

With this change, the item transforms will be computed differently. For
example, if the parent item is transformed, a child's transform will
look as follows

  [Parent's translation][Parent's transform][Child's translation]

instead of

  [Parent's translation][Child's translation][Parent's transform]

In the future, I'd like to get rid of the Item::setTransform() call in
OpenGLWindow::performPaint() and have either AnimationEffect or
libkwineffects call Item::setTransform().

BUG: 440201