dav1dplay: allow resizing the window

libplacebo v66 got helper functions that make preserving the aspect
ratio in this case trivial. But we still need to make sure to clear the
FBO to black if the image doesn't cover it fully.

examples/dp_renderer_placebo.c

@@ -80,7 +80,7 @@ static Dav1dPlayRendererPrivateContext*
     placebo_renderer_create_common(int window_flags)
 {
     // Create Window
-    SDL_Window *sdlwin = dp_create_sdl_window(window_flags);
+    SDL_Window *sdlwin = dp_create_sdl_window(window_flags | SDL_WINDOW_RESIZABLE);
     if (sdlwin == NULL)
         return NULL;
 
@@ -326,6 +326,12 @@ static void placebo_render(void *cookie, const Dav1dPlaySettings *settings)
         .len = 0,
     };
 
+#if PL_API_VER >= 66
+    pl_rect2df_aspect_copy(&target.dst_rect, &rd_priv_ctx->image.src_rect, 0.0);
+    if (pl_render_target_partial(&target))
+        pl_tex_clear(rd_priv_ctx->gpu, target.fbo, (float[4]){ 0.0 });
+#endif
+
     if (!pl_render_image(rd_priv_ctx->renderer, &rd_priv_ctx->image, &target, &render_params)) {
         fprintf(stderr, "Failed rendering frame!\n");
         pl_tex_clear(rd_priv_ctx->gpu, target.fbo, (float[4]){ 1.0 });
@@ -366,6 +372,7 @@ static int placebo_upload_image(void *cookie, Dav1dPicture *dav1d_pic,
         .num_planes = 3,
         .width      = width,
         .height     = height,
+        .src_rect   = {0, 0, width, height},
 
         .repr = {
             .bits = {