Render to texture must use resolution lower than application window size.

**Description**
Rendering to a texture, as in the [render-to-texture example](https://github.com/armory3d/armory_examples/tree/master/render_to_texture), _silently_ requires that the texture be smaller than the application window, because it reuses many of the buffers in the current render path.

Granted, in many cases this won't be noticeable; if the pixel of the texture is smaller than its rendered on-screen size. From a cursory glance through the rendering code, this appears to be partially iron's fault as well, given at least some of the "render size" comes from the app window. It means, though, that you can't render anything above the size of the **physical monitor**, making certain runtime/virtual texture applications impossible (for example, high resolution screenshots in a "photo mode").

**Test File**
[render_to_texture.zip](https://github.com/armory3d/armory/files/5539724/render_to_texture.zip)

Modified render-to-texture example. Note that the resolution of the backing texture has been set to 8k, at which resolution it should be noticeably smoother when rendered behind Suzanne.

Start with the linked render-to-texture example and the attached render_to_texture.zip, reproducing the 8k backing-texture case against the current application window. Trace the render-size and buffer handling described in the issue; done means render-to-texture no longer silently requires a texture smaller than the window or physical monitor.

Written by the indexing model from the issue text.