Began work on 'screen.qc' -- some code shared between the MENU and CLIENT qc
programs for detailing with the screen (especially size / aspect ratio stuff).
Data : added 'prydoncursor001.tga' and '..002.tga' -- cursor images which
look like a glove.  Images created by "yd", sourced from OGA, license CC0.
Screen lib : bit of a rethink on determining virtual screen size, now we
always set 'scr_height' to be same as real video height.
Data : removed place-holder 'gfx/cursor_arrow.tga' image
Screen lib : set virtual height to a fixed value (768).
Menu : use the new 'screen.qc' code.
Client : use new 'screen.qc' lib for screen sizing, and experimented with
drawing an inventory box.