Add subdirectories to engine/ similar to pokecrystal

engine/gfx/screen_effects.asm

@@ -0,0 +1,71 @@
+; b = new colour for BG colour 0 (usually white) for 4 frames
+ChangeBGPalColor0_4Frames:
+	call GetPredefRegisters
+	ld a, [rBGP]
+	or b
+	ld [rBGP], a
+	ld c, 4
+	call DelayFrames
+	ld a, [rBGP]
+	and %11111100
+	ld [rBGP], a
+	ret
+
+PredefShakeScreenVertically:
+; Moves the window down and then back in a sequence of progressively smaller
+; numbers of pixels, starting at b.
+	call GetPredefRegisters
+	ld a, 1
+	ld [wDisableVBlankWYUpdate], a
+	xor a
+.loop
+	ld [$ff96], a
+	call .MutateWY
+	call .MutateWY
+	dec b
+	ld a, b
+	jr nz, .loop
+	xor a
+	ld [wDisableVBlankWYUpdate], a
+	ret
+
+.MutateWY
+	ld a, [$ff96]
+	xor b
+	ld [$ff96], a
+	ld [rWY], a
+	ld c, 3
+	jp DelayFrames
+
+PredefShakeScreenHorizontally:
+; Moves the window right and then back in a sequence of progressively smaller
+; numbers of pixels, starting at b.
+	call GetPredefRegisters
+	xor a
+.loop
+	ld [$ff97], a
+	call .MutateWX
+	ld c, 1
+	call DelayFrames
+	call .MutateWX
+	dec b
+	ld a, b
+	jr nz, .loop
+
+; restore normal WX
+	ld a, 7
+	ld [rWX], a
+	ret
+
+.MutateWX
+	ld a, [$ff97]
+	xor b
+	ld [$ff97], a
+	bit 7, a
+	jr z, .skipZeroing
+	xor a ; zero a if it's negative
+.skipZeroing
+	add 7
+	ld [rWX], a
+	ld c, 4
+	jp DelayFrames