keep DEPTH from causing compile error in GLES2

(cherry picked from commit 5056b4a02c)

drivers/gles2/shaders/scene.glsl

@@ -1466,6 +1466,9 @@ void main() {
 	float anisotropy = 0.0;
 	vec2 anisotropy_flow = vec2(1.0, 0.0);
 	float sss_strength = 0.0; //unused
+	// gl_FragDepth is not available in GLES2, so writing to DEPTH is not converted to gl_FragDepth by Godot compiler resulting in a
+	// compile error because DEPTH is not a variable.
+	float m_DEPTH = 0.0;
 
 	float alpha = 1.0;
 	float side = 1.0;