Fix incorrect text size with downscaled preview

Use either scene render size or fixed preview scale factor.

Previously scene render size was used as baseline value for text size
correction. This is incorrect.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D9563

source/blender/sequencer/intern/effects.c

@@ -3902,7 +3902,7 @@ static ImBuf *do_text_effect(const SeqRenderData *context,
   /* Compensate text size for preview render size. */
   proxy_size_comp = context->scene->r.size / 100.0;
   if (context->preview_render_size != SEQ_RENDER_SIZE_SCENE) {
-    proxy_size_comp *= BKE_sequencer_rendersize_to_scale_factor(context->preview_render_size);
+    proxy_size_comp = BKE_sequencer_rendersize_to_scale_factor(context->preview_render_size);
   }
 
   /* set before return */