### Godot version

3.5.1 custom build

### System information

N/A

### Issue description

When using Line2D, I noticed a discrepancy with UVs depending on the end cap mode.

Here are some examples using a tiled repeating texture:

### No capping ✔️ 
![image](https://user-images.githubusercontent.com/95356449/218230984-e5d507d4-7af7-4967-aee7-79c8daa206ff.png)

### Begin cap: Box ✔️ 
![image](https://user-images.githubusercontent.com/95356449/218231092-311370cc-7ff8-4699-976b-c4581eaada0a.png)

### End cap: Box ❌ 
![image](https://user-images.githubusercontent.com/95356449/218231131-00ddf4f7-2a9a-4740-a445-a8601ab026e1.png)


### Steps to reproduce

Create a Line2D
Add a texture with repeating enabled
Set end cap mode to Box

### Minimal reproduction project

not needed, bugfix pr incoming
} else {
			pos_up0 = pos1 + u1 * hw * width_factor;
			pos_down0 = pos1 - u1 * hw * width_factor;
		}
		
		
		if (current_joint_mode != Line2D::LINE_JOINT_SHARP) {
			/* ________________ cbegin
			 *               / \
			 *              /   \
			 * ____________/_ _ _\ cend
			 *             |     |
			 *             |     |
			 *             |     |
			 */
			Vector2 cbegin, cend;
			if (orientation == UP) {
				cbegin = pos_down1;
				cend = pos_down0;
			} else {
				cbegin = pos_up1;
				cend = pos_up0;
			}
			if (current_joint_mode == Line2D::LINE_JOINT_BEVEL) {
				strip_add_tri(cend, orientation);
			} else if (current_joint_mode == Line2D::LINE_JOINT_ROUND) {
				Vector2 vbegin = cbegin - pos1;
				Vector2 vend = cend - pos1;
				strip_add_arc(pos1, vbegin.angle_to(vend), orientation);
			}