AtlasTexture Fix calculating rects when flipping

[kleonc]

Fixes #70178.

The issue was likely caused/revealed by #56795/#56796, quoting myself:

(...) As a result an AtlasTexture being a source atlas of another AtlasTexture could have resulted in incorrect rendering. This should be fixed now (assuming other methods work as they're supposed to).

Seems like this wasn't the case, offset calculations when flipping the texture (when destination rect has negative size):

godot/scene/resources/texture.cpp

Lines 1607 to 1618 in d44d0cc

	Vector2 ofs = (src_c.position - src.position);
	if (scale.x < 0) {
	float mx = (margin.size.width - margin.position.x);
	mx -= margin.position.x;
	ofs.x = -(ofs.x + mx);
	}
	if (scale.y < 0) {
	float my = margin.size.height - margin.position.y;
	my -= margin.position.y;
	ofs.y = -(ofs.y + my);
	}

don't make much sense to me. When not flipping the offset is correctly the difference of the positions of the clipped source rect and the non-clipped source rect. When flipping the offset (for the axes being flipped) should be the difference between the rect.ends of the same rects instead (this should already account for the margins as the source rect is clipped to the region), meaning just adding the difference of the sizes of these rects should be enough. And that's what this PR does, the rest are cosmetic changes to clarify the code a little (renaming/removing some variables).

[akien-mga]

Thanks!

[timothyqiu]

Cherry-picked for 3.6.

[akien-mga]

Cherry-picked for 3.5.2.