Initial SDL_gpu support for dialogs.
SDL_Color-parametered overload for ttext::set_foreground_color.
Only redraw invalidated parts of the panel.

Fixes flickering buttons.
Fix compilation.
Add function draw_text_to_texture.
SDL_gpu implementation for the unit preview pane.
Don't share attributes among different sdl::timage instances.
Print FPS numbers to cerr in SDL_gpu builds.
Draw terrain overlays on the minimap.
Fix compilation.