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Adjust MAXRATE_1080P.

This value was originally set in response to requests from the hardware
team before levels were properly defined for VP9.

Even if a level is not specified for an encode, it imposes a maximum
frame size for videos of dimensions <= 1080P.  For larger formats the
limit was set at 250 bits per MB.

This patch modifies the limit to be more in line with the requirements
specified for level 4 (max rate for a 4 frame group of 16 Mbits).  If a lower
level is specified at encode time and this mandates a smaller maximum frame
size then the level requirement will still take precedence.

Increasing this value allows for some slide shows or very low motion clips
to code a better quality key frame.

Change-Id: Ic08e0e09c8a918077152190c59732b9a1c049787

--- a/vp9/encoder/vp9_ratectrl.c

+++ b/vp9/encoder/vp9_ratectrl.c

@@ -31,10 +31,13 @@

 #include "vp9/encoder/vp9_encodemv.h"

 #include "vp9/encoder/vp9_ratectrl.h"

-// Max rate target for 1080P and below encodes under normal circumstances

-// (1920 * 1080 / (16 * 16)) * MAX_MB_RATE bits per MB

+// Max rate per frame for 1080P and below encodes if no level requirement given.

+// For larger formats limit to MAX_MB_RATE bits per MB

+// 4Mbits is derived from the level requirement for level 4 (1080P 30) which

+// requires that HW can sustain a rate of 16Mbits over a 4 frame group.

+// If a lower level requirement is specified then this may over ride this value.

 #define MAX_MB_RATE 250

-#define MAXRATE_1080P 2025000

+#define MAXRATE_1080P 4000000

 #define DEFAULT_KF_BOOST 2000

 #define DEFAULT_GF_BOOST 2000

@@ -1913,12 +1916,12 @@

       VPXMAX(rc->min_frame_bandwidth, FRAME_OVERHEAD_BITS);

   // A maximum bitrate for a frame is defined.

-  // The baseline for this aligns with HW implementations that

-  // can support decode of 1080P content up to a bitrate of MAX_MB_RATE bits

-  // per 16x16 MB (averaged over a frame). However this limit is extended if

-  // a very high rate is given on the command line or the the rate cannnot

-  // be acheived because of a user specificed max q (e.g. when the user

-  // specifies lossless encode.

+  // However this limit is extended if a very high rate is given on the command

+  // line or the the rate cannnot be acheived because of a user specificed max q

+  // (e.g. when the user specifies lossless encode).

+  //

+  // If a level is specified that requires a lower maximum rate then the level

+  // value take precedence.

   vbr_max_bits =

       (int)(((int64_t)rc->avg_frame_bandwidth * oxcf->two_pass_vbrmax_section) /

             100);