Make use_opengl local

The flag is used only locally, there is no need to store it in the
screen structure.

app/src/screen.c

@@ -280,8 +280,8 @@ screen_init_rendering(struct screen *screen, const char *window_title,
     LOGI("Renderer: %s", renderer_name ? renderer_name : "(unknown)");
 
     // starts with "opengl"
-    screen->use_opengl = renderer_name && !strncmp(renderer_name, "opengl", 6);
+    bool use_opengl = renderer_name && !strncmp(renderer_name, "opengl", 6);
-    if (screen->use_opengl) {
+    if (use_opengl) {
         struct sc_opengl *gl = &screen->gl;
         sc_opengl_init(gl);
 
@@ -444,7 +444,6 @@ update_texture(struct screen *screen, const AVFrame *frame) {
             frame->data[2], frame->linesize[2]);
 
     if (screen->mipmaps) {
-        assert(screen->use_opengl);
         SDL_GL_BindTexture(screen->texture, NULL, NULL);
         screen->gl.GenerateMipmap(GL_TEXTURE_2D);
         SDL_GL_UnbindTexture(screen->texture);

app/src/screen.h

@@ -16,7 +16,6 @@ struct screen {
     SDL_Window *window;
     SDL_Renderer *renderer;
     SDL_Texture *texture;
-    bool use_opengl;
     struct sc_opengl gl;
     struct size frame_size;
     struct size content_size; // rotated frame_size
@@ -41,7 +40,6 @@ struct screen {
     .window = NULL, \
     .renderer = NULL, \
     .texture = NULL, \
-    .use_opengl = false, \
     .gl = {0}, \
     .frame_size = { \
         .width = 0, \