Add iNes Mapper 003 (CNROM) Support

Add support for iNES Mapper 003, which is a very simple mapper
only swiching out CHR chunks.

There is 16kB of fixed space mapped from $C000-FFFF and,
optionally 16kB more of fixed space from $8000-$BFFF (if not,
this area is mirrored)

The CHR ROM from $0000-$1FFF is banked in 8k chunks, up to four.
Some games look for bus contention, it is not implemented.

src/cartridge.cpp

@@ -3,6 +3,7 @@
 #include "cpu.hpp"
 #include "mappers/mapper0.hpp"
 #include "mappers/mapper1.hpp"
+#include "mappers/mapper3.hpp"
 #include "mappers/mapper4.hpp"
 #include "ppu.hpp"
 #include "cartridge.hpp"
@@ -52,6 +53,7 @@ void load(const char* fileName)
     {
         case 0:  mapper = new Mapper0(rom); break;
         case 1:  mapper = new Mapper1(rom); break;
+        case 3:  mapper = new Mapper3(rom); break;
         case 4:  mapper = new Mapper4(rom); break;
     }
 

src/mappers/mapper3.cpp

@@ -0,0 +1,50 @@
+#include "ppu.hpp"
+#include "mappers/mapper3.hpp"
+
+/* Based off of https://wiki.nesdev.com/w/index.php/INES_Mapper_003 */
+
+/* Apply the registers state */
+void Mapper3::apply()
+{
+    if (PRG_size_16k)
+    {
+    /*
+     * mirror the bottom on the top
+     * 0x8000 - 0xBFFF ==
+     * 0xC000 - 0xFFFF
+     */
+        map_prg<16>(0,0);
+        map_prg<16>(1,0);
+    }
+    else
+    {
+        /* no mirroring */
+        map_prg<16>(0,0);
+        map_prg<16>(1,1);
+    }
+
+    /* 8k bankswitched CHR */
+    map_chr<8>(0, regs[0] & 0b11);
+
+    /* mirroring is based on the header (soldered) */
+    set_mirroring(vertical_mirroring?PPU::VERTICAL:PPU::HORIZONTAL);
+}
+
+u8 Mapper3::write(u16 addr, u8 v)
+{
+    /* check for bus contingency? */
+
+    /* chr bank switching */
+    if (addr & 0x8000)
+    {
+      regs[0] = v;
+      apply();
+    }
+    return v;
+}
+
+u8 Mapper3::chr_write(u16 addr, u8 v)
+{
+    return chr[addr] = v;
+}
+

src/mappers/mapper3.hpp

@@ -0,0 +1,24 @@
+#pragma once
+#include "mapper.hpp"
+
+
+class Mapper3 : public Mapper
+{
+    u8 regs[1];
+    bool vertical_mirroring;
+    bool PRG_size_16k;
+    void apply();
+
+    public:
+    Mapper3(u8* rom) : Mapper(rom)
+    {
+        PRG_size_16k = rom[4] == 1;
+        vertical_mirroring = rom[7] & 0x01;
+        regs[0] = 0;
+        apply();
+    }
+
+    u8 write(u16 addr, u8 v);
+    u8 chr_write(u16 addr, u8 v);
+};
+