feat(video): track background and sprite mosaic state

Add vertical mosaic counters and background/sprite sampling regressions.

tests/mosaic.rs

@@ -0,0 +1,47 @@
+use gba_rust::{
+    io::*,
+    memory::{Memory, MemoryError, OAM_START as OAM, PALETTE_START as PAL, VRAM_START as VRAM},
+    video::{rgb555_to_rgb888 as rgb, Framebuffer, HEIGHT, WIDTH},
+};
+
+#[path = "mosaic/backgrounds.rs"]
+mod backgrounds;
+#[path = "mosaic/demo.rs"]
+mod demo;
+#[path = "mosaic/objects.rs"]
+mod objects;
+
+fn memory() -> Memory {
+    let mut m = Memory::new(vec![]).unwrap();
+    for index in 0..128 {
+        m.write16(OAM + index * 8, 0x200).unwrap();
+    }
+    for index in 0..256 {
+        let color = (index & 31) | (((index >> 3) & 31) << 5) | (((index * 7) & 31) << 10);
+        m.write16(PAL + index * 2, color as u16).unwrap();
+        m.write16(PAL + 0x200 + index * 2, color as u16).unwrap();
+    }
+    m
+}
+
+fn render(m: &Memory) -> Framebuffer {
+    let mut f = Framebuffer::default();
+    m.render_frame(&mut f).unwrap();
+    f
+}
+
+fn object(m: &mut Memory, index: u32, a: u16, b: u16, c: u16) {
+    m.write16(OAM + index * 8, a).unwrap();
+    m.write16(OAM + index * 8 + 2, b).unwrap();
+    m.write16(OAM + index * 8 + 4, c).unwrap();
+}
+
+fn fill(m: &mut Memory, address: u32, bytes: u32, value: u16) {
+    for offset in (0..bytes).step_by(2) {
+        m.write16(address + offset, value).unwrap();
+    }
+}
+
+fn at(frame: &Framebuffer, x: usize, y: usize) -> u32 {
+    frame.pixels()[y * WIDTH + x]
+}