Fix parsing of G bit in VP9 scalability structure

[robfig]

Previously it would erroneously look at some of the reserved fields.

[Sean-Der]

Thank you @robfig sorry this took so long :(

[Sean-Der]

@robfig Would you mind looking at the test TestVP9Packet_Unmarshal/ScalabilityMissingReferenceIndices it starts failing with this change.

I should have time during the holidays though! I will try to come back to this.

[robfig]

I just exported this fix from our own fork and don't have any particular knowledge about it, so I'll have to tag in @cptpcrd

[cptpcrd]

@robfig Would you mind looking at the test TestVP9Packet_Unmarshal/ScalabilityMissingReferenceIndices it starts failing with this change.

It appears that this test (and also ScalabilityMissingNG and ScalabilityMissingTemporalLayerIDs) are not actually setting the G bit - they're setting one of the reserved bits, and relying the current erroneous parsing of the G bit:

$ git diff
diff --git a/codecs/vp9_packet.go b/codecs/vp9_packet.go
index 63f05e1..2704097 100644
--- a/codecs/vp9_packet.go
+++ b/codecs/vp9_packet.go
@@ -436,6 +436,9 @@ func (p *VP9Packet) parseSSData(packet []byte, pos int) (int, error) {
        p.NS = packet[pos] >> 5
        p.Y = packet[pos]&0x10 != 0
        p.G = packet[pos]&0x8 != 0
+       if packet[pos]&0x7 != 0 {
+               panic("Reserved bits set")
+       }
        pos++
 
        NS := p.NS + 1
diff --git a/codecs/vp9_packet_test.go b/codecs/vp9_packet_test.go
index 15205ef..1f58824 100644
--- a/codecs/vp9_packet_test.go
+++ b/codecs/vp9_packet_test.go
@@ -169,21 +169,6 @@ func TestVP9Packet_Unmarshal(t *testing.T) {
                                Payload: []byte{},
                        },
                },
-               "ScalabilityStructureReserved": {
-                       b: []byte{
-                               0x0A,
-                               (1 << 5) | (0 << 4) | (0 << 3) | (1 << 2) | (1 << 1) | 1, // NS:1 Y:0 G:0, reserved fields set to 1
-                       },
-                       pkt: VP9Packet{
-                               B:       true,
-                               V:       true,
-                               NS:      1,
-                               Y:       false,
-                               G:       false,
-                               NG:      0,
-                               Payload: []byte{},
-                       },
-               },
                "ScalabilityStructure_ShortPacket0": {
                        b:   []byte{0x0A, 0x10},
                        err: errShortPacket,
$ go test ./codecs
--- FAIL: TestVP9Packet_Unmarshal (0.00s)
    --- FAIL: TestVP9Packet_Unmarshal/ScalabilityMissingReferenceIndices (0.00s)
panic: Reserved bits set [recovered]
	panic: Reserved bits set
...

I've pushed a change to the tests which fixes this.