VP9 decoder: Sets thread count based on resolution, reinit on change.

Previously, number of decoder threads for VP9 were always set to 8 but
with a cap at number of cores. This was done since we "can't know" the
resolution that will be used.

With this change, we now intialize the number of threads based on
resolution given in InitDecode(). If a resolution change happens in
flight, it requires a keyframe. We therefore parse the header from
any key frame and if it has a new resolution, we re-initialize the
decoder.

The number of threads used is based on pixel count. We set one thread
as target for 1280x720, and scale up lineraly from there. The 8-thread
cap is gone, but still limit it core count.

This means for instance: 1 <= 720p, 2 for 1080p, 4 for 1440p, 9 for 4K.

Bug: webrtc:11551
Change-Id: I14c169a6c651c50bd1b870c4b22bc4495c8448fd
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/174460
Commit-Queue: Erik Språng <sprang@webrtc.org>
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31507}

modules/video_coding/utility/vp9_uncompressed_header_parser.h

@@ -13,6 +13,7 @@
 
 #include <stddef.h>
 #include <stdint.h>
+#include "absl/types/optional.h"
 
 namespace webrtc {
 
@@ -22,6 +23,65 @@ namespace vp9 {
 // Returns true on success, false otherwise.
 bool GetQp(const uint8_t* buf, size_t length, int* qp);
 
+// Bit depth per channel. Support varies by profile.
+enum class BitDept : uint8_t {
+  k8Bit = 8,
+  k10Bit = 10,
+  k12Bit = 12,
+};
+
+enum class ColorSpace : uint8_t {
+  CS_UNKNOWN = 0,    // Unknown (in this case the color space must be signaled
+                     // outside the VP9 bitstream).
+  CS_BT_601 = 1,     // CS_BT_601 Rec. ITU-R BT.601-7
+  CS_BT_709 = 2,     // Rec. ITU-R BT.709-6
+  CS_SMPTE_170 = 3,  // SMPTE-170
+  CS_SMPTE_240 = 4,  // SMPTE-240
+  CS_BT_2020 = 5,    // Rec. ITU-R BT.2020-2
+  CS_RESERVED = 6,   // Reserved
+  CS_RGB = 7,        // sRGB (IEC 61966-2-1)
+};
+
+enum class ColorRange {
+  kStudio,  // Studio swing:
+            // For BitDepth equals 8:
+            //     Y is between 16 and 235 inclusive.
+            //     U and V are between 16 and 240 inclusive.
+            // For BitDepth equals 10:
+            //     Y is between 64 and 940 inclusive.
+            //     U and V are between 64 and 960 inclusive.
+            // For BitDepth equals 12:
+            //     Y is between 256 and 3760.
+            //     U and V are between 256 and 3840 inclusive.
+  kFull     // Full swing; no restriction on Y, U, V values.
+};
+
+enum class YuvSubsampling {
+  k444,
+  k440,
+  k422,
+  k420,
+};
+
+struct FrameInfo {
+  int profile = 0;  // Profile 0-3 are valid.
+  bool show_frame = false;
+  bool error_resilient = false;
+  BitDept bit_detph = BitDept::k8Bit;
+  ColorSpace color_space = ColorSpace::CS_UNKNOWN;
+  ColorRange color_range;
+  YuvSubsampling sub_sampling;
+  int frame_width = 0;
+  int frame_height = 0;
+  int render_width = 0;
+  int render_height = 0;
+};
+
+// Parses frame information for a VP9 key-frame or all-intra frame from a
+// bitstream. Returns nullopt on failure or if not a key-frame.
+absl::optional<FrameInfo> ParseIntraFrameInfo(const uint8_t* buf,
+                                              size_t length);
+
 }  // namespace vp9
 
 }  // namespace webrtc