plugins/screencast: Fix hidpi region screencasting
Currently hidpi region screencasting is broken because output textures are inappropriately scaled.
The orthographic projection is set up with logical pixels, but the remaining rendering code uses "1 / m_scale" scale factor, which is wrong.
In order to fix hidpi and also simplify rendering logic, this change makes rendering code use logical coordinate system. It's okay to do in screencasting because we don't need to worry about snapping to the pixel grid.