Emulation improvements

• CPU: Fix fir_mov logic for MULS instruction
• PPU: Improve blending over transparent pixels
• PPU: Draw blended sprites in depth layer order
• SPU: Support wave-in software mix channel
• SPG200: Add watchdog emulation

UI changes

• Add SPU output visualization window
• Add memory editor
• Add support for selecting and loading ROMs from UI
• Add setting for allowing background gamepad input
• Fix border artifacts in bilinear fullscreen mode
• Ignore keyboard presses for controller input if ImGui window has focus
• Add icon, FreeDesktop desktop entry file
• Add About window
• Disable imgui.ini