Fix some Node3DEditor snapping issues

[kleonc]

Fixes #79862.

There was a discrepancy between what's saved in the plugin's state and where it gets loaded to:

godot/editor/plugins/node_3d_editor_plugin.cpp

Lines 5860 to 5862 in dd74ffd

	d["translate_snap"] = get_translate_snap();
	d["rotate_snap"] = get_rotate_snap();
	d["scale_snap"] = get_scale_snap();

godot/editor/plugins/node_3d_editor_plugin.cpp

Lines 5940 to 5950 in dd74ffd

	if (d.has("translate_snap")) {
	snap_translate_value = d["translate_snap"];
	}
	if (d.has("rotate_snap")) {
	snap_rotate_value = d["rotate_snap"];
	}
	if (d.has("scale_snap")) {
	snap_scale_value = d["scale_snap"];
	}

All get_translate_snap/get_rotate_snap/get_scale_snap were reading the values from the snap dialog's LineEdits instead of using plugin's snap_translate_value/snap_rotate_value/snap_scale_value (which are being updated according to these LineEdits on pressing OK / confirming in the snap dialog). Also what's worse (given these were the values being saved), these methods return smaller snap values when Shift is being pressed.

godot/editor/plugins/node_3d_editor_plugin.cpp

Lines 8889 to 8920 in dd74ffd

	double Node3DEditor::get_translate_snap() const {
	double snap_value;
	if (Input::get_singleton()->is_key_pressed(Key::SHIFT)) {
	snap_value = snap_translate->get_text().to_float() / 10.0;
	} else {
	snap_value = snap_translate->get_text().to_float();
	}
	return snap_value;
	}
	double Node3DEditor::get_rotate_snap() const {
	double snap_value;
	if (Input::get_singleton()->is_key_pressed(Key::SHIFT)) {
	snap_value = snap_rotate->get_text().to_float() / 3.0;
	} else {
	snap_value = snap_rotate->get_text().to_float();
	}
	return snap_value;
	}
	double Node3DEditor::get_scale_snap() const {
	double snap_value;
	if (Input::get_singleton()->is_key_pressed(Key::SHIFT)) {
	snap_value = snap_scale->get_text().to_float() / 2.0;
	} else {
	snap_value = snap_scale->get_text().to_float();
	}
	return snap_value;
	}

Meaning if Shift was pressed when plugin's state was being saved then values smaller than the actual snap values were being saved into a file. Then when loading they'd be loaded as the actual snap values. Everytime this happened the snap values were becoming smaller and smaller.

For reproducing #79862 it was enough for me to create two 3D scenes and keep switching between them (by pressing their tabs) while keeping Shift pressed.

This PR makes get_translate_snap/get_rotate_snap/get_scale_snap properly use snap_translate_value/snap_rotate_value/snap_scale_value, and makes snap_translate_value/snap_rotate_value/snap_scale_value be what's saved in the plugin's state.

Since get_translate_snap/get_rotate_snap/get_scale_snap no longer read values from LineEdits they were changed to return real_t instead of double, as that's the type of snap_translate_value/snap_rotate_value/snap_scale_value. See #49783 (comment) for why they were returning doubles before.
If we'd want snapping to always operate on doubles then what would need to be changed are snap_translate_value/snap_rotate_value/snap_scale_value. I could make such changes if that's desired.

---

Fixes #39507.

godot/editor/plugins/node_3d_editor_plugin.cpp

Lines 4885 to 4886 in dd74ffd

	angle = Math::rad_to_deg(angle) + snap * 0.5; //else it won't reach +180
	angle -= Math::fmod(angle, snap);

Math::fmod would return NaN for snap == 0.0. Besides, this calculation doesn't correctly snap for negative angles anyway.
Fixed it to use Math::snapped which handles these cases properly.

[aaronfranke]

I have not tested this but it looks good.

[akien-mga]

Thanks!

[rcorre]

Wow, great find! This was driving me crazy!