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/common/onyx.h

diff --git a/vp8/common/onyx.h b/vp8/common/onyx.h
index 426b8fc..c586d0d 100644
--- a/vp8/common/onyx.h
+++ b/vp8/common/onyx.h
@@ -109,6 +109,7 @@
         int noise_sensitivity;   // parameter used for applying pre processing blur: recommendation 0
         int Sharpness;          // parameter used for sharpening output: recommendation 0:
         int cpu_used;
+        unsigned int rc_max_intra_bitrate_pct;
 
         // mode ->
         //(0)=Realtime/Live Encoding. This mode is optimized for realtim encoding (for example, capturing