Add rc_max_intra_bitrate_pct control

Adds a control to limit the maximum size of a keyframe, as a function of
the per-frame bitrate. See this thread[1] for more detailed discussion:

[1]: http://groups.google.com/a/webmproject.org/group/codec-devel/browse_thread/thread/271b944a5e47ca38

Change-Id: I7337707642eb8041d1e593efc2edfdf66db02a94

vp8/encoder/onyx_if.c

@@ -2704,6 +2704,19 @@ static int pick_frame_size(VP8_COMP *cpi)
         }
     }
 
+    /* Apply limits on keyframe target.
+     *
+     * TODO: move this after consolidating
+     * vp8_calc_iframe_target_size() and vp8_calc_auto_iframe_target_size()
+     */
+    if (cm->frame_type == KEY_FRAME && cpi->oxcf.rc_max_intra_bitrate_pct)
+    {
+        unsigned int max_rate = cpi->av_per_frame_bandwidth
+                                * cpi->oxcf.rc_max_intra_bitrate_pct / 100;
+
+        if (cpi->this_frame_target > max_rate)
+            cpi->this_frame_target = max_rate;
+    }
     return 1;
 }