Added SDL2 display code to sdl2.h

Taken from the SDL2 branch.

It's not enough to work as is, though, still missing some pieces like rg_queue implementation.

components/retro-go/drivers/display/sdl2.h

@@ -1,28 +1,68 @@
 #include <SDL2/SDL.h>
 
+static SDL_Window *window;
+static SDL_Surface *surface, *canvas;
+static int win_left, win_top, win_width, win_height, cursor;
+static uint16_t lcd_buffer[LCD_BUFFER_LENGTH];
+
 static void lcd_init(void)
 {
+    window = SDL_CreateWindow("Retro-Go", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, RG_SCREEN_WIDTH, RG_SCREEN_HEIGHT, 0);
+    surface = SDL_GetWindowSurface(window);
+    canvas = SDL_CreateRGBSurfaceWithFormat(0, display.screen.width, display.screen.height, 16, SDL_PIXELFORMAT_RGB565);
 }
 
 static void lcd_deinit(void)
 {
 }
 
+static void lcd_set_window(int left, int top, int width, int height)
+{
+    int right = left + width - 1;
+    int bottom = top + height - 1;
+    if (left < 0 || top < 0 || right >= RG_SCREEN_WIDTH || bottom >= RG_SCREEN_HEIGHT)
+        RG_LOGW("Bad lcd window (x0=%d, y0=%d, x1=%d, y1=%d)\n", left, top, right, bottom);
+    win_left = left;
+    win_top = top;
+    win_width = width;
+    win_height = height;
+    cursor = 0;
+}
+
 static void lcd_set_backlight(float percent)
 {
 }
 
 static inline uint16_t *lcd_get_buffer(size_t length)
 {
-    return (void *)0;
+    return lcd_buffer;
 }
 
 static inline void lcd_send_buffer(uint16_t *buffer, size_t length)
 {
+    int bpp = canvas->format->BytesPerPixel;
+    int pitch = canvas->pitch;
+    void *pixels = canvas->pixels;
+    for (size_t i = 0; i < length; ++i) {
+        int real_top = win_top + (cursor / win_width);
+        int real_left = win_left + (cursor % win_width);
+        if (real_top >= RG_SCREEN_HEIGHT || real_left >= RG_SCREEN_WIDTH)
+            return;
+        uint16_t *dst = (void*)pixels + (real_top * pitch) + (real_left * bpp);
+        uint16_t pixel = buffer[i];
+        *dst = ((pixel & 0xFF) << 8) | ((pixel & 0xFF00) >> 8);;
+        cursor++;
+    }
 }
 
 static void lcd_sync(void)
 {
+    SDL_Event user_event;
+    user_event.type = SDL_USEREVENT;
+    user_event.user.code = 2;
+    user_event.user.data1 = NULL;
+    user_event.user.data2 = NULL;
+    SDL_PushEvent(&user_event);
 }
 
 const rg_display_driver_t rg_display_driver_sdl2 = {