Use window size for offscreen texture size in screenshot effect

Effect used the display size which caused windows being larger than
either width or height not to be captured correctly.

Now it uses the window size for the offscreen texture which also requires
to adjust the modelviewprojection matrix.

As an unrelated change the pointer to GLRenderTarget and GLTexture are
changed to a QScopedPointer and some variables which got moved around
are consted.

BUG: 312851
FIXED-IN: 4.11
REVIEW: 108258