[Impeller] Removed requirement for multisample buffers from egl setup. (flutter/engine#40944)

I reviewed the code and I don't think this requirement is necessary.
Offscreen MSAA is already guarded by a flag and looking at the opengles
code it doesn't seem to be using multisample buffers.

When removing it the emulator no longer crashes and behaves identically
to on device (which is rendering incorrect colors and crashing when
pressing a the counter button).

fixes https://github.com/flutter/flutter/issues/105323

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- [x] I listed at least one issue that this PR fixes in the description
above.
- [ ] I added new tests to check the change I am making or feature I am
adding, or Hixie said the PR is test-exempt. See [testing the engine]
for instructions on writing and running engine tests.
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engine/src/flutter/impeller/toolkit/egl/display.cc

@@ -141,9 +141,9 @@ std::unique_ptr<Config> Display::ChooseConfig(ConfigDescriptor config) const {
     if (sample_count > 1) {
       attributes.push_back(EGL_SAMPLE_BUFFERS);
       attributes.push_back(1);
+      attributes.push_back(EGL_SAMPLES);
+      attributes.push_back(sample_count);
     }
-    attributes.push_back(EGL_SAMPLES);
-    attributes.push_back(sample_count);
   }
 
   // termination sentinel must be present.

engine/src/flutter/shell/platform/android/android_surface_gl_impeller.cc

@@ -98,10 +98,19 @@ AndroidSurfaceGLImpeller::AndroidSurfaceGLImpeller(
   desc.samples = impeller::egl::Samples::kFour;
 
   desc.surface_type = impeller::egl::SurfaceType::kWindow;
-  auto onscreen_config = display->ChooseConfig(desc);
+  std::unique_ptr<impeller::egl::Config> onscreen_config =
+      display->ChooseConfig(desc);
   if (!onscreen_config) {
-    FML_DLOG(ERROR) << "Could not choose onscreen config.";
-    return;
+    // Fallback for Android emulator.
+    desc.samples = impeller::egl::Samples::kOne;
+    onscreen_config = display->ChooseConfig(desc);
+    if (onscreen_config) {
+      FML_LOG(INFO) << "Warning: This device doesn't support MSAA for onscreen "
+                       "framebuffers. Falling back to a single sample.";
+    } else {
+      FML_DLOG(ERROR) << "Could not choose onscreen config.";
+      return;
+    }
   }
 
   desc.surface_type = impeller::egl::SurfaceType::kPBuffer;