Use StoreVisualX() all over the code

Since we already have the StoreVisualX() helper, use it all over the
place instead of setting LastVisualX directly.

This will allow us to add more logic to StoreVisualX() add let this
extra logic apply everywhere automatically.

internal/buffer/cursor.go

@@ -100,7 +100,7 @@ func (c *Cursor) GetCharPosInLine(b []byte, visualPos int) int {
 // Start moves the cursor to the start of the line it is on
 func (c *Cursor) Start() {
 	c.X = 0
-	c.LastVisualX = c.GetVisualX()
+	c.StoreVisualX()
 }
 
 // StartOfText moves the cursor to the first non-whitespace rune of
@@ -131,7 +131,7 @@ func (c *Cursor) IsStartOfText() bool {
 // End moves the cursor to the end of the line it is on
 func (c *Cursor) End() {
 	c.X = util.CharacterCount(c.buf.LineBytes(c.Y))
-	c.LastVisualX = c.GetVisualX()
+	c.StoreVisualX()
 }
 
 // CopySelection copies the user's selection to either "primary"