Added impellerc example for auditing fragment shaders (#12613)

_Description of what this PR is changing or adding, and why:_

Adds low level example of how to audit the output of impellerc.

_Issues fixed by this PR (if any):_

flutter/flutter#171471

_PRs or commits this PR depends on (if any):_

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Author: gaaclarke

src/content/ui/design/graphics/fragment-shaders.md

@@ -258,6 +258,29 @@ with the index starting over at 0.
 
 Any float uniforms that are left uninitialized will default to `0.0`.
 
+The reflection data generated by the Flutter's shader compiler can be audited
+with the following commands in order to see things like uniform offsets.
+
+```shell
+cd $FLUTTER
+# Generate the .sl file.
+`find bin/ -name impellerc` \
+  --runtime-stage-metal \
+  --iplr \
+  --input=path/to/myshader.frag \
+  --sl=foo.sl \
+  --spirv=foo.spirv \
+  --include=engine/src/flutter/impeller/compiler/shader_lib/ \
+  --input-type=frag
+# Convert the .sl file to .json
+flatc \
+  --json \
+  ./engine/src/flutter/impeller/runtime_stage/runtime_stage.fbs \
+  -- ./foo.sl
+# View results
+cat foo.json
+```
+
 #### Current position
 
 The shader has access to a `varying` value that contains the local coordinates for