Add caught icon to battle HUD for already‐owned Pokémon

The battle HUD is drawn in DrawEnemyHUDAndHPBar: from engine/battle/core.asm.

You can add the "Pokéball" tile to gfx/font/font_battle_extra.png by replacing an unused tile, such as the unused Japanese opening quote tile (looks like an upside-down L); the second tile in the second row. This ends up as tile ID $72 when it is loaded into VRAM. If you replace a different unused tile, then be sure to use the correct tile ID in the code below.

...
DrawEnemyHUDAndHPBar:
	xor a
	ldh [hAutoBGTransferEnabled], a
	hlcoord 0, 0
	lb bc, 4, 12
	call ClearScreenArea
	callfar PlaceEnemyHUDTiles
+	push hl
+	ld a, [wEnemyMonSpecies2]
+	ld [wPokedexNum], a
+	callfar IndexToPokedex
+	ld a, [wPokedexNum]
+	dec a
+	ld c, a
+	ld b, FLAG_TEST
+	ld hl, wPokedexOwned
+	predef FlagActionPredef
+	ld a, c
+	and a
+	jr z, .notOwned
+	hlcoord 1, 1
+	ld [hl], $72 ; replace this with your Poké Ball icon or other character
+.notOwned
+	pop hl
	ld de, wEnemyMonNick
	hlcoord 1, 0
	call CenterMonName
	call PlaceString
	hlcoord 4, 1
	push hl
	inc hl
...