ili9341: add EnableTEOutput to be able to sync drawing with VSYNC

Many displays don't have the TE pin exposed. But those that do have the
pin (for example, the PyPortal) can use it to synchronize writing a new
image to the display. When implemented correctly, tearing can be avoided
entirely.

This commit also changes the LCD refresh direction to either
top-to-bottom or left-to-right depending on the rotation. Previously it
might refresh from right-to-left or bottom-to-top. This has little
impact on code that doesn't use the TE line, but code that does now only
needs to worry about two cases (top-to-bottom and left-to-right) instead
of four.
(Unfortunately, it appears that the hardware doesn't support changing
the major LCD refresh order so code that wants to do tear-free rendering
still needs to care about these two cases).

ili9341/registers.go

@@ -41,6 +41,8 @@ const (
 
 	PTLAR    = 0x30 ///< Partial Area
 	VSCRDEF  = 0x33 ///< Vertical Scrolling Definition
+	TEOFF    = 0x34 ///< TEOFF: Tearing Effect Line OFF
+	TEON     = 0x35 ///< TEON: Tearing Effect Line ON
 	MADCTL   = 0x36 ///< Memory Access Control
 	VSCRSADD = 0x37 ///< Vertical Scrolling Start Address
 	PIXFMT   = 0x3A ///< COLMOD: Pixel Format Set