Make offscreen OpenGL-Rendering mode efficient

[pasbi]

Currently, the size of the texture that is being rendered is only affected by the current zoom and the object size.
That is, as you zoom closer to the object, the texture grows and grows.

That is not necessary and inefficient: The visible size of the texture does not need to be larger than the viewport size (in pixels).
I.e., if the zoom is deep enough, it suffices to render just a part of the object.

That'd limit the texture size to the viewport size without decreasing quality.
Note that the application currently crashes, if the offscreen texture becomes too large (the assertion in the Texture-constructor fails).