GUAC-936: Use initial resolution for all future resizing of display.

This change modifies the manner in which effective display resolution is decided and stored:

1. The requested DPI is maintained throughout the session, along with the requested width/height, rather than overwriting the client optimal resolution and dimensions.
2. If the DPI is not forced via the "dpi" parameter, heuristics determine a "reasonable" resolution based on resulting screen area.
3. Future size requests are adjusted to remote coordinates, such that the effective DPI of the session is consistent.