package/flutter-engine: use impeller rendering engine

From https://docs.flutter.dev/perf/impeller:

Impeller provides a new rendering runtime for Flutter. The Flutter
team’s believes this solves Flutter’s early-onset jank issue. Impeller
precompiles a smaller, simpler set of shaders at Engine build time so
they don’t compile at runtime.

Impeller has the following objectives:

  - Predictable performance: Impeller compiles all shaders reflection
    and offline at build time. It builds all pipeline state objects
    upfront.  The engine controls caching and caches explicitly.

  - Instrumentable: Impeller tags and labels all graphics resources
    like textures, and buffers. It can capture and persist animations
    to disk without affecting per-frame rendering performance.

  - Portable: Flutter doesn’t tie Impeller to a specific client
    rendering API.  You can author shaders once and convert them to
    backend-specific formats as necessary.

  - Leverages modern graphics APIs: Impeller uses, but doesn’t depend
    on, features available in modern APIs like Metal and Vulkan.

  - Leverages concurrency: Impeller can distribute single-frame
    workloads across multiple threads if necessary.

Signed-off-by: Adam Duskett <adam.duskett@amarulasolutions.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/flutter-engine/flutter-engine.mk

@@ -124,6 +124,7 @@ endif
 
 ifeq ($(BR2_PACKAGE_HAS_LIBGLES),y)
 FLUTTER_ENGINE_DEPENDENCIES += libgles
+FLUTTER_ENGINE_CONF_OPTS += --enable-impeller-opengles
 endif
 
 ifeq ($(BR2_PACKAGE_LIBGLFW),y)
@@ -142,7 +143,7 @@ endif
 
 # There is no --disable-vulkan option
 ifeq ($(BR2_PACKAGE_MESA3D_VULKAN_DRIVER),y)
-FLUTTER_ENGINE_CONF_OPTS += --enable-vulkan
+FLUTTER_ENGINE_CONF_OPTS += --enable-vulkan --enable-impeller-vulkan
 endif
 
 ifeq ($(BR2_PACKAGE_XORG7)$(BR2_PACKAGE_LIBXCB),yy)