[master] Mode -fullscreen broken, mode -root half-broken(?)

[hartwork]

While #18 fixed -root for invocation by XScreensaver, -fullscreen is still broken and -root still behaves differently on Git master compared to 2.08:

2.08 behavior:

• ./src/fireflies -root (outside of XScreensaver) renders "somewhere invisible"
• ./src/fireflies -root (inside XScreensaver) renders fullscreen
• ./src/fireflies -fullscreen renders fullscreen, no window decorations (hard to exit)
• ./src/fireflies renders to a non-fullscreen window with decorations (constant 1024x768over here)

Git master behavior:

• ./src/fireflies -root (outside of XScreensaver) renders to a fullscreen window with window decorations?
• ./src/fireflies -root (inside XScreensaver) renders fullscreen
• ./src/fireflies -fullscreen renders to a non-fullscreen window with decorations (constant 1280x720)
• ./src/fireflies renders to a non-fullscreen window with decorations (constant 1280x720 over here)

[hartwork]

PS: I can see in 0f73116 that we had this code for flag -fullscreen with canvas SDL…

    if (full_screen)
       flags |= SDL_FULLSCREEN;

…while neither of the two remaining canvases GLX and GLUT act upon flag full_screen.