Fix warnings about uninitialized vars [-Wsometimes-uninitialized]

Fixes the following Clang 7 warnings:
```
drivers/gles3/rasterizer_scene_gles3.cpp:1260:10: warning: variable 'target' is used uninitialized whenever switch case is taken [-Wsometimes-uninitialized]
drivers/gles3/rasterizer_scene_gles3.cpp:1280:10: warning: variable 'target' is used uninitialized whenever switch case is taken [-Wsometimes-uninitialized]
drivers/gles3/rasterizer_scene_gles3.cpp:1281:10: warning: variable 'target' is used uninitialized whenever switch case is taken [-Wsometimes-uninitialized]
drivers/gles3/rasterizer_scene_gles3.cpp:1282:10: warning: variable 'target' is used uninitialized whenever switch case is taken [-Wsometimes-uninitialized]
drivers/gles3/rasterizer_scene_gles3.cpp:1286:5: warning: variable 'target' is used uninitialized whenever switch default is taken [-Wsometimes-uninitialized]
editor/editor_properties_array_dict.cpp:982:15: warning: variable 'change_index' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]
editor/editor_properties.cpp:2655:6: warning: variable 'lt' is used uninitialized whenever switch default is taken [-Wsometimes-uninitialized]
editor/settings_config_dialog.cpp:367:11: warning: variable 'current_search_box' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]
```

drivers/gles3/rasterizer_scene_gles3.cpp

@@ -1182,7 +1182,7 @@ bool RasterizerSceneGLES3::_setup_material(RasterizerStorageGLES3::Material *p_m
 
 		glActiveTexture(GL_TEXTURE0 + i);
 
-		GLenum target;
+		GLenum target = GL_TEXTURE_2D;
 		GLuint tex = 0;
 
 		RasterizerStorageGLES3::Texture *t = storage->texture_owner.getptr(textures[i]);

editor/editor_properties.cpp

@@ -2638,7 +2638,7 @@ bool EditorInspectorDefaultPlugin::parse_property(Object *p_object, Variant::Typ
 
 			} else if (p_hint == PROPERTY_HINT_LAYERS_2D_PHYSICS || p_hint == PROPERTY_HINT_LAYERS_2D_RENDER || p_hint == PROPERTY_HINT_LAYERS_3D_PHYSICS || p_hint == PROPERTY_HINT_LAYERS_3D_RENDER) {
 
-				EditorPropertyLayers::LayerType lt;
+				EditorPropertyLayers::LayerType lt = EditorPropertyLayers::LAYER_RENDER_2D;
 				switch (p_hint) {
 					case PROPERTY_HINT_LAYERS_2D_RENDER:
 						lt = EditorPropertyLayers::LAYER_RENDER_2D;

editor/editor_properties_array_dict.cpp

@@ -969,7 +969,7 @@ void EditorPropertyDictionary::update_property() {
 				pc->add_child(add_vbox);
 			}
 			prop->set_object_and_property(object.ptr(), prop_name);
-			int change_index;
+			int change_index = 0;
 
 			if (i < amount) {
 				String cs = key.get_construct_string();

editor/settings_config_dialog.cpp

@@ -361,7 +361,7 @@ void EditorSettingsDialog::_tabs_tab_changed(int p_tab) {
 void EditorSettingsDialog::_focus_current_search_box() {
 
 	Control *tab = tabs->get_current_tab_control();
-	LineEdit *current_search_box;
+	LineEdit *current_search_box = NULL;
 	if (tab == tab_general)
 		current_search_box = search_box;
 	else if (tab == tab_shortcuts)