feat: make O'Mesh on Impeller great again

[renancaraujo]

Description

Since the instability on Impeller seems to be specifically when we use .drawVertices with a FragmentShader, I figured if we pre-render all gradient patches in a temporary canvas, record it into a ui.Image instance and then use an ImageShader instead of a FragmentShader, impeller won't panic.

Of course, this has some performance overhead, but since #10, O'Mesh's performance has been so good, that it's unnoticeable in almost all hardware I tested (mac m1 and iPad a14).

But to be safe, I added two flags:

void main(){
  // Do this when you are opting out of impeller
  enableOMeshImpellerCompatibility = false;
  runApp();
}

void main(){
  // Do this when you are opting in into android impeller
  enableOMeshImpellerCompatibilityOnAndroid = true;
  runApp();
}

Breaking

The impellerCompatibilityMode option available on the widgets are not necessary anymore.

(Required) Link to the issue

#9

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