Revert "Set GLArea.has-alpha = true (#54053)" (#54190)

Reason for revert: Regression + no tests. Must have been at the end of a long day, cause I missed the missing test and glaring warning about that.

This reverts commit 0844c0a.

Re-opens flutter/flutter#152154

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Author: cbracken

shell/platform/linux/fl_renderer.cc

@@ -445,7 +445,7 @@ void fl_renderer_render(FlRenderer* self, int width, int height) {
 
   g_return_if_fail(FL_IS_RENDERER(self));
 
-  glClearColor(0.0, 0.0, 0.0, 0.0);
+  glClearColor(0.0, 0.0, 0.0, 1.0);
   glClear(GL_COLOR_BUFFER_BIT);
 
   if (priv->has_gl_framebuffer_blit) {

shell/platform/linux/fl_view.cc

@@ -743,7 +743,6 @@ static void fl_view_init(FlView* self) {
                            self);
 
   self->gl_area = GTK_GL_AREA(gtk_gl_area_new());
-  gtk_gl_area_set_has_alpha(self->gl_area, TRUE);
   gtk_widget_show(GTK_WIDGET(self->gl_area));
   gtk_container_add(GTK_CONTAINER(self->event_box), GTK_WIDGET(self->gl_area));