vf_gpu: initialize depth=1 on target textures

According to internal documentation, 2D textures should have d=1.
This fixes the correctness of the API usage, not any bug.

video/filter/vf_gpu.c

@@ -196,6 +196,7 @@ static struct mp_image *gpu_render_frame(struct mp_filter *f, struct mp_image *i
             .downloadable = true,
             .w = w,
             .h = h,
+            .d = 1,
             .render_dst = true,
         };