GBA: Refine multiboot detection (fixes #2192)

CHANGES

@@ -47,6 +47,7 @@ Misc:
  - Debugger: Save and restore CLI history
  - GB Video: Add default SGB border
  - GBA: Automatically skip BIOS if ROM has invalid logo
+ - GBA: Refine multiboot detection (fixes mgba.io/i/2192)
  - GBA DMA: Enhanced logging (closes mgba.io/i/2454)
  - GBA Video: Implement layer placement for OpenGL renderer (fixes mgba.io/i/1962)
  - mGUI: Add margin to right-aligned menu text (fixes mgba.io/i/871)

src/gba/gba.c

@@ -672,13 +672,23 @@ bool GBAIsMB(struct VFile* vf) {
 	}
 
 	uint32_t pc = GBA_MB_MAGIC_OFFSET;
+	int wramAddrs = 0;
+	int wramLoads = 0;
+	int romAddrs = 0;
+	int romLoads = 0;
 	int i;
 	for (i = 0; i < 80; ++i) {
 		if (vf->read(vf, &signature, sizeof(signature)) != sizeof(signature)) {
 			break;
 		}
 		pc += 4;
 		LOAD_32(opcode, 0, &signature);
+		if ((opcode & ~0x7FF) == BASE_WORKING_RAM) {
+			++wramAddrs;
+		}
+		if ((opcode & ~0x7FF) == BASE_CART0) {
+			++romAddrs;
+		}
 		ARMDecodeARM(opcode, &info);
 		if (info.mnemonic != ARM_MN_LDR) {
 			continue;
@@ -700,10 +710,19 @@ bool GBAIsMB(struct VFile* vf) {
 				break;
 			}
 			if ((immediate & ~0x7FF) == BASE_WORKING_RAM) {
-				return true;
+				++wramLoads;
+			}
+			if ((immediate & ~0x7FF) == BASE_CART0) {
+				++romLoads;
 			}
 		}
 	}
+	if (romLoads + romAddrs > 2) {
+		return false;
+	}
+	if (wramLoads + wramAddrs) {
+		return true;
+	}
 	// Found a libgba-linked cart...these are a bit harder to detect.
 	return false;
 }