Initial SDL_gpu support in the game display.
Rename ttexture to timage.
Move sdl/texture.?pp to sdl/image.?pp.
Make timage purely a wrapper class.

No longer simulate that different instances always handle different
textures.
Add clone() function to timage.
SDL_gpu implementation for draw_panel.
Disable map borders (temporary).
Invalidate whole screen when scrolling on SDL_gpu builds.

A (hopefully) temporary fix for rendering issues when scrolling.
Don't blit terrain on panels in SDL_gpu builds.
Remove an unnecessary flip().
Dummy SDL_gpu implementation for image::get_lighted_image.
Normalize zoom factor to 72 for SCALED_TO_HEX images.
Re-enable map borders.
Remove obsolete todo comment.
Change default color alpha for SDL_gpu builds.

In SDL_gpu, 255 is 'opaque'.
SDL_gpu-compatible minimap drawer function.
Fix compilation.
Fix minimap positioning.
SDL_gpu implementation for reports.
Reomve unnecessary flip.