Extract texture management to a separate file

Move texture management out of screen.c, which already handles the
window and the rendering.

This paves the way to implement optional software filtering (swscale)
properly, as an alternative to mipmaps (which are not available
everywhere).

app/meson.build

@@ -11,6 +11,7 @@ src = [
     'src/event_converter.c',
     'src/file_handler.c',
     'src/fps_counter.c',
+    'src/frame_texture.c',
     'src/input_manager.c',
     'src/opengl.c',
     'src/receiver.c',