Fix clear color's alpha value will affects 2D editor in Compatibility mode

When the rt's transparency is false, force the alpha of the clear color to one

(cherry picked from commit e21a485dbc3d0540652c4527b30d059e7924eccd)

drivers/gles3/rasterizer_canvas_gles3.cpp

@@ -2105,7 +2105,7 @@ void RasterizerCanvasGLES3::canvas_begin(RID p_to_render_target, bool p_to_backb
 
 	if (render_target && render_target->clear_requested) {
 		const Color &col = render_target->clear_color;
-		glClearColor(col.r, col.g, col.b, col.a);
+		glClearColor(col.r, col.g, col.b, render_target->is_transparent ? col.a : 1.0f);
 
 		glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);
 		render_target->clear_requested = false;