Support for Libretro

Related to #14 (closed)

frontends/libretro/src/consts.rs

+pub const RETRO_API_VERSION: u32 = 1;
+pub const REGION_NTSC: u32 = 0;
+
+pub const RETRO_ENVIRONMENT_SET_PIXEL_FORMAT: u32 = 10;
+
+pub const RETRO_PIXEL_FORMAT_0RGB1555: usize = 0;
+pub const RETRO_PIXEL_FORMAT_XRGB8888: usize = 1;
+pub const RETRO_PIXEL_FORMAT_RGB565: usize = 2;
+
+pub const RETRO_MEMORY_SAVE_RAM: u32 = 0;
+pub const RETRO_MEMORY_SYSTEM_RAM: u32 = 0;
+
+pub const RETRO_DEVICE_JOYPAD: usize = 1;
+
+pub const RETRO_DEVICE_ID_JOYPAD_B: isize = 0;
+pub const RETRO_DEVICE_ID_JOYPAD_Y: isize = 1;
+pub const RETRO_DEVICE_ID_JOYPAD_SELECT: isize = 2;
+pub const RETRO_DEVICE_ID_JOYPAD_START: isize = 3;
+pub const RETRO_DEVICE_ID_JOYPAD_UP: isize = 4;
+pub const RETRO_DEVICE_ID_JOYPAD_DOWN: isize = 5;
+pub const RETRO_DEVICE_ID_JOYPAD_LEFT: isize = 6;
+pub const RETRO_DEVICE_ID_JOYPAD_RIGHT: isize = 7;
+pub const RETRO_DEVICE_ID_JOYPAD_A: isize = 8;
+pub const RETRO_DEVICE_ID_JOYPAD_X: isize = 9;
+pub const RETRO_DEVICE_ID_JOYPAD_L: isize = 10;
+pub const RETRO_DEVICE_ID_JOYPAD_R: isize = 11;
+pub const RETRO_DEVICE_ID_JOYPAD_L2: isize = 12;
+pub const RETRO_DEVICE_ID_JOYPAD_R2: isize = 13;
+pub const RETRO_DEVICE_ID_JOYPAD_L3: isize = 14;
+pub const RETRO_DEVICE_ID_JOYPAD_R3: isize = 15;