Various fix for GUI related issue

Fixes #201 (closed) and #191 (closed) There was a inconstistency between the viewport height (minimal 512) and the render target height (user specified, potentially < 512). There is now a minimal height of 128 for the viewport when using the interactive mode. The render target will follow this behavior to avoid previous issue.

#95 (closed) The window was initialized using the specified render target size, regardless of screen resolution. The behavior is now:

• fetch screen resolution.
• clamp render target if above (including gui elements).
• create window to fit within the screen resolution.