[Reverse-engineering] [th01] .BOS bitplane pointers

No idea why ZUN split the 6 slots into 4 with a regular alpha plane,
and 2 with a negated alpha plane. The latter are only used for Sariel's
animations.

Part of P0095, funded by Yanga.

th01/formats/bos.hpp

@@ -0,0 +1,32 @@
+/// Uncompressed 16-color 16w×h sprite format
+/// -----------------------------------------
+#define BOS_IMAGES_PER_SLOT 8
+
+struct bos_image_t {
+	dots16_t *B;
+	dots16_t *R;
+	dots16_t *G;
+	dots16_t *E;
+	dots16_t *alpha;
+};
+
+struct bos_t {
+	bos_image_t image[BOS_IMAGES_PER_SLOT];
+};
+
+// Regular
+// -------
+#define BOS_SLOT_COUNT 4
+extern bos_t bos_images[BOS_SLOT_COUNT];
+// -------
+
+// Fast
+// ----
+// These… only have functions to direct byte-aligned blitting onto page 0, in
+// exchange for the alpha plane being pre-negated at load time? No idea why.
+// That 1-instruction negation is certainly not what makes the original code
+// slow.
+#define BOS_FAST_SLOT_COUNT 2
+extern bos_t bos_fast_images[BOS_FAST_SLOT_COUNT];
+// ----
+/// -----------------------------------------

th01/formats/bos[bss].asm

@@ -0,0 +1,19 @@
+BOS_SLOT_COUNT = 4
+BOS_FAST_SLOT_COUNT = 2
+BOS_IMAGES_PER_SLOT = 8
+
+bos_image_t struc
+	BOS_B	dd ?
+	BOS_R	dd ?
+	BOS_G	dd ?
+	BOS_E	dd ?
+	BOS_alpha	dd ?
+bos_image_t ends
+
+bos_t struc
+	BOS_image	bos_image_t BOS_IMAGES_PER_SLOT dup (<?>)
+bos_t ends
+
+public _bos_images, _bos_fast_images
+_bos_images	bos_t BOS_SLOT_COUNT dup(<?>)
+_bos_fast_images	bos_t BOS_FAST_SLOT_COUNT dup(<?>)