Fix compilation of bitmapfont sample

Samples/basic/bitmapfont/src/FontEngineInterfaceBitmap.cpp

@@ -105,7 +105,7 @@ int FontEngineInterfaceBitmap::GetStringWidth(FontFaceHandle handle, const Strin
 }
 
 int FontEngineInterfaceBitmap::GenerateString(FontFaceHandle handle, FontEffectsHandle /*font_effects_handle*/, const String& string,
-	const Vector2f& position, const Colourb& colour, GeometryList& geometry)
+	const Vector2f& position, const Colourb& colour, float /*opacity*/, GeometryList& geometry)
 {
 	auto handle_bitmap = reinterpret_cast<FontFaceBitmap*>(handle);
 	return handle_bitmap->GenerateString(string, position, colour, geometry);

Samples/basic/bitmapfont/src/FontEngineInterfaceBitmap.h

@@ -87,7 +87,8 @@ public:
 	int GetStringWidth(FontFaceHandle handle, const String& string, Character prior_character = Character::Null) override;
 
 	/// Called by RmlUi when it wants to retrieve the geometry required to render a single line of text.
-	int GenerateString(FontFaceHandle face_handle, FontEffectsHandle font_effects_handle, const String& string, const Vector2f& position, const Colourb& colour, GeometryList& geometry) override;
+	int GenerateString(FontFaceHandle face_handle, FontEffectsHandle font_effects_handle, const String& string, const Vector2f& position,
+		const Colourb& colour, float opacity, GeometryList& geometry) override;
 
 	/// Called by RmlUi to determine if the text geometry is required to be re-generated.eometry.
 	int GetVersion(FontFaceHandle handle) override;