Move texture atlas from customcontext to qt quick

ssh://codereview.qt-project.org:29418/playground/scenegraph.git's customcontext contains atlas code which should be moved to the qtquick module to facilitate better batching with the new renderer.

QQuickWindow::createTextureFromImage() should get an additional option to make the atlas opt-in as atlas textures can easily break when the user assumes a 0.0-1.0 texture coordinate range.