fix hidpi?

The window dimensions are integers, so resizing to fit the content may
not be exact.

When computing the optimal size, it could cause to reduce the width and
height alternatively by few pixels, making the "optimal size" unstable.

For the same reason, during rendering, the content size could be
different from the optimal window size, leading to black borders
(supposed to be removed in optimal size).

To avoid this problem, always choose the same dimension if the choice to
resize the width or the height is in the error range.

app/src/screen.c

@@ -342,6 +342,8 @@ screen_init_rendering(struct screen *screen, const char *window_title,
         return false;
     }
 
+    update_content_rect(screen);
+
     screen->windowed_window_size = window_size;
 
     return true;
@@ -417,6 +419,7 @@ prepare_for_frame(struct screen *screen, struct size new_frame_size) {
 
         screen->frame_size = new_frame_size;
         screen->content_size = new_content_size;
+        update_content_rect(screen);
 
         LOGI("New texture: %" PRIu16 "x%" PRIu16,
                      screen->frame_size.width, screen->frame_size.height);
@@ -455,7 +458,6 @@ screen_update_frame(struct screen *screen, struct video_buffer *vb) {
         mutex_unlock(vb->mutex);
         return false;
     }
-    update_content_rect(screen);
     update_texture(screen, frame);
     mutex_unlock(vb->mutex);
 
@@ -599,9 +601,14 @@ screen_convert_to_frame_coords(struct screen *screen, int32_t x, int32_t y) {
     int32_t w = screen->content_size.width;
     int32_t h = screen->content_size.height;
 
-    // scale
-    x = (x - screen->rect.x) * w / screen->rect.w;
-    y = (y - screen->rect.y) * h / screen->rect.h;
+    // take the HiDPI scaling (dw/ww and dh/wh) into account
+    int ww, wh, dw, dh;
+    SDL_GetWindowSize(screen->window, &ww, &wh);
+    SDL_GL_GetDrawableSize(screen->window, &dw, &dh);
+
+    // scale (64 bits for intermediate multiplications)
+    x = (int64_t) (x - screen->rect.x) * w * dw / (screen->rect.w * ww);
+    y = (int64_t) (y - screen->rect.y) * h * dh / (screen->rect.h * wh);
 
     // rotate
     struct point result;