[Impeller] Match position attribute name across runtime effect stages

impeller/compiler/shader_lib/flutter/runtime_effect.glsl

@@ -7,6 +7,9 @@
 
 #if defined(IMPELLER_GRAPHICS_BACKEND)
 
+// Note: The GLES backend uses name matching for attribute locations. This name
+// must match the name of the attribute output in:
+// impeller/entity/shaders/runtime_effect.vert
 in vec2 _fragCoord;
 vec2 FlutterFragCoord() {
   return _fragCoord;

impeller/entity/shaders/runtime_effect.vert

@@ -10,9 +10,12 @@ uniform VertInfo {
 vert_info;
 
 in vec2 position;
-out vec2 v_position;
+// Note: The GLES backend uses name matching for attribute locations. This name
+// must match the name of the attribute input in:
+// impeller/compiler/shader_lib/flutter/runtime_effect.glsl
+out vec2 _fragCoord;
 
 void main() {
   gl_Position = vert_info.mvp * vec4(position, 0.0, 1.0);
-  v_position = position;
+  _fragCoord = position;
 }