[Impeller] Add FAQ entry clarifying Skia usage and Web support. (#34076)

Author: chinmaygarde

impeller/docs/faq.md

@@ -10,3 +10,40 @@
       `./flutter/tools/gn`. Only do this if you frequently work with Playgrounds
       and don't want to have to set the flags manually. Also, it would be a bad
       idea to set this environment variable on CI.
+* Does Impeller use Skia for rendering?
+  * No. Impeller has no direct dependencies on Skia.
+  * When running with Impeller, Flutter does not create a Skia graphics context.
+  * However, while Impeller still performs text rendering, text layout and
+    shaping needs to be done by a separate component. This component happens to
+    the SkParagraph which is part of Skia.
+  * Similarly, Impeller does not perform image decompression. Flutter uses a
+    standard set of codecs wrapped by Skia before querying the system supplied
+    image formats.
+  * So, while Impeller does not use nor is it a wrapper for Skia, some Skia
+    components are still used by Flutter when rendering using Impeller.
+* Is Impeller going to be supported on the Web?
+  * The current priority for Impeller is to be amazing on all platforms targeted
+    by the C++ engine. This includes iOS, Android, desktops, and, all Embedder
+    API users. This would be by building Metal, Open GL, Open GL ES, and, Vulkan
+    rendering backends.
+  * The Open GL ES backend ought to work fine to target WebGL/WebGL2 and the
+    team can fix any issues found in such uses of the backend.
+  * However, in Flutter, Impeller sits behind the Display List interface in the
+    C++ engine. Display lists apply optimizations to the Flutter rendering
+    intent. But, more importantly for Impeller, they also provide a generic
+    interface with the ability to specify "dispatchers" to different rendering
+    packages. Today, the engine has Skia and Impeller dispatchers for display
+    lists.
+  * The web engine is unique in that it doesn't use any C++ engine components.
+    This includes the display lists mechanism. Instead, it interfaces directly
+    with Skia via the CanvasKit package.
+  * Updating the web engine to interface directly with Impeller is a non-goal at
+    this time. It is a significant undertaking (compared to a flag to swap
+    dispatchers that already exists) and also bypasses display list
+    optimizations.
+  * For this added implementation complexity, Web support has not been a
+    priority at this time for the small team working on Impeller.
+  * We are aware that these priorities might change in the future. There have
+    been sanity checks to ensure that the Impeller API can be ported to WASM and
+    also that Impeller shaders can be [compiled to WGSL](https://github.com/chinmaygarde/wgsl_sandbox)
+    for eventual WebGPU support.