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/src/frame_texture.h

@@ -0,0 +1,33 @@
+#ifndef SC_FRAME_TEXTURE_H
+#define SC_FRAME_TEXTURE_H
+
+#include "common.h"
+
+#include <stdbool.h>
+#include <SDL2/SDL.h>
+#include <libavformat/avformat.h>
+
+#include "coords.h"
+#include "opengl.h"
+
+struct sc_frame_texture {
+    SDL_Renderer *renderer; // owned by struct screen
+
+    bool mipmaps;
+    struct sc_opengl gl;
+
+    SDL_Texture *texture;
+    struct size texture_size;
+};
+
+bool
+sc_frame_texture_init(struct sc_frame_texture *ftex, SDL_Renderer *renderer,
+                      bool mipmaps, struct size initial_size);
+
+void
+sc_frame_texture_destroy(struct sc_frame_texture *ftex);
+
+bool
+sc_frame_texture_update(struct sc_frame_texture *ftex, const AVFrame *frame);
+
+#endif