Use L1T3 for screen sharing (#858)

Mirroring JS (and Flutter) workaround for:

```cpp
   if (codec.mode == VideoCodecMode::kScreensharing) {
     // TODO(bugs.webrtc.org/11999): Compose names of the structures when they
     // are implemented.
     return nullptr;  // ← Returns nullptr regardless of flexible mode!
   }
```

100% reproducible.

Author: pblazej

.changes/screensharing-vp9

@@ -0,0 +1 @@
+patch type="fixed" "Screen sharing not publishing frames with VP9/AV1 codecs"

Sources/LiveKit/Support/Utils.swift

@@ -225,7 +225,8 @@ class Utils: Loggable {
         if let videoCodec, videoCodec.isSVC {
             // SVC mode
             log("Using SVC mode")
-            return [RTC.createRtpEncodingParameters(encoding: encoding, scalabilityMode: .L3T3_KEY)]
+            // VP9/AV1 with screen sharing requires single spatial layer
+            return [RTC.createRtpEncodingParameters(encoding: encoding, scalabilityMode: isScreenShare ? .L1T3 : .L3T3_KEY)]
         } else if !publishOptions.simulcast {
             // Not-simulcast mode
             log("Simulcast not enabled")

Sources/LiveKit/Types/ScalabilityMode.swift

@@ -21,11 +21,13 @@ public enum ScalabilityMode: Int {
     case L3T3 = 1
     case L3T3_KEY = 2
     case L3T3_KEY_SHIFT = 3
+    case L1T3 = 4
 }
 
 public extension ScalabilityMode {
     static func fromString(_ rawString: String?) -> ScalabilityMode? {
         switch rawString {
+        case "L1T3": .L1T3
         case "L3T3": .L3T3
         case "L3T3_KEY": .L3T3_KEY
         case "L3T3_KEY_SHIFT": .L3T3_KEY_SHIFT
@@ -35,13 +37,19 @@ public extension ScalabilityMode {
 
     var rawStringValue: String {
         switch self {
+        case .L1T3: "L1T3"
         case .L3T3: "L3T3"
         case .L3T3_KEY: "L3T3_KEY"
         case .L3T3_KEY_SHIFT: "L3T3_KEY_SHIFT"
         }
     }
 
-    var spatial: Int { 3 }
+    var spatial: Int {
+        switch self {
+        case .L1T3: 1
+        case .L3T3, .L3T3_KEY, .L3T3_KEY_SHIFT: 3
+        }
+    }
 
     var temporal: Int { 3 }
 }