Export UV Map has poor UV line quality

[SamVanheer]

The Export UV Map dialog has poor UV line quality. This is caused by the use of Qt's painting functionality which has trouble creating solid lines.

Once the program is using shaders a framebuffer can be created with render-to-texture support to reuse the OpenGL rendering logic for this feature.

See also:
https://www.khronos.org/opengl/wiki/Framebuffer_Object
http://www.opengl-tutorial.org/intermediate-tutorials/tutorial-14-render-to-texture/
https://learnopengl.com/Advanced-OpenGL/Framebuffers