void PushQuad ( Texture texture, SpriteVertex v0, SpriteVertex v1, SpriteVertex v2, SpriteVertex v3 )
{
texture = texture ?? defaultTexture;
if ( groups.Count == 0 || groups[groups.Count-1].Texture != texture ) {
groups.Add( new Group { Texture = texture, StartSprite = spriteCount } );
}
groups[groups.Count-1].SpriteCount++;
spriteCount++;
vertices.Add( v0 );
vertices.Add( v1 );
vertices.Add( v2 );
vertices.Add( v0 );
vertices.Add( v2 );
vertices.Add( v3 );
dirty = true;
}