Transparent video read/write support (#145)

* Add support for reading transparent Matroska and implement alpha side data & decode

* Few fixes

* Implement alpha encoding

* Gracefully handle inability to acquire WebGL context, properly clean up WebGL contexts

* Improve touch device detection

* Test test

* Test test #2

* Test test 3

* Test test 4

* Test test 5

* Test test 6

* Test test 7

* Test test 8

* Test test 9

* Test test 10

* Test test 11

* Test test 12

* Test test 13

* Test test 14

* Test test 15

* Test test 16

* Test test 17

* Test test 18

* Test test 19

* Test test 20

* Fix type errors, improve MetadataTags docs

* Do a bunch of docs work

* Test test?

* "Unexpected only modifier 🤓"

* Add InputVideoTrack.canBeTransparent()

* Some clean-up

* Adjust CI to be less spammy in PRs

* Fix broken license headers

src/input-track.ts

@@ -192,6 +192,7 @@ export interface InputVideoTrackBacking extends InputTrackBacking {
 	getCodedHeight(): number;
 	getRotation(): Rotation;
 	getColorSpace(): Promise<VideoColorSpaceInit>;
+	canBeTransparent(): Promise<boolean>;
 	getDecoderConfig(): Promise<VideoDecoderConfig | null>;
 }
 
@@ -260,6 +261,11 @@ export class InputVideoTrack extends InputTrack {
 			|| (colorSpace.matrix as string) === 'bt2020-ncl';
 	}
 
+	/** Checks if this track may contain transparent samples with alpha data. */
+	canBeTransparent() {
+		return this._backing.canBeTransparent();
+	}
+
 	/**
 	 * Returns the [decoder configuration](https://www.w3.org/TR/webcodecs/#video-decoder-config) for decoding the
 	 * track's packets using a [`VideoDecoder`](https://developer.mozilla.org/en-US/docs/Web/API/VideoDecoder). Returns
@@ -312,7 +318,10 @@ export class InputVideoTrack extends InputTrack {
 			return null;
 		}
 
-		return determineVideoPacketType(this, packet);
+		const decoderConfig = await this.getDecoderConfig();
+		assert(decoderConfig);
+
+		return determineVideoPacketType(this.codec, decoderConfig, packet.data);
 	}
 }