Use higher level helpers instead of raw defines

[AntonioND]

manual/manuscript/backgrounds.md

Lines 271 to 274 in 6a9c46b

	```C++
	{{#include snippets/backgrounds/1/main.cpp:dma_channel}}
	{{#include snippets/backgrounds/1/main.cpp:display_backgrounds}}
	```

manual/manuscript/snippets/backgrounds/1/main.cpp

Lines 114 to 139 in 6a9c46b

	void displayStarField() {
	dmaCopyHalfWords(DMA_CHANNEL, starFieldBitmap, /* This variable is generated
	* for us by grit. */
	(uint16 *)BG_BMP_RAM(0), /* Our address for main
	* background 3 */
	starFieldBitmapLen); /* This length (in bytes) is generated
	* from grit. */
	}
	void displayPlanet() {
	dmaCopyHalfWords(DMA_CHANNEL, planetBitmap, /* This variable is generated for
	* us by grit. */
	(uint16 *)BG_BMP_RAM(8), /* Our address for main
	* background 2 */
	planetBitmapLen); /* This length (in bytes) is generated
	* from grit. */
	}
	void displaySplash() {
	dmaCopyHalfWords(DMA_CHANNEL, splashBitmap, /* This variable is generated for
	* us by grit. */
	(uint16 *)BG_BMP_RAM_SUB(0), /* Our address for sub
	* background 3 */
	splashBitmapLen); /* This length (in bytes) is generated
	* from grit. */
	}