add missing sdl.zig

in_one_weekend/src/sdl.zig

@@ -0,0 +1,11 @@
+pub const c = @cImport({ @cInclude("SDL.h"); });
+const Color = @import("vec3.zig").Color;
+
+pub fn setSurfacePixel(surface: *c.SDL_Surface, i: usize, j: usize, color: Color) void {
+    const bytePerPixel = 4;
+    const index = i * bytePerPixel + j * @intCast(usize, surface.*.pitch);
+    const target_pixel = @ptrToInt(surface.*.pixels) + index;
+    @intToPtr(*u8, target_pixel + 0).* = @floatToInt(u8, color.z * 255.99);
+    @intToPtr(*u8, target_pixel + 1).* = @floatToInt(u8, color.y * 255.99);
+    @intToPtr(*u8, target_pixel + 2).* = @floatToInt(u8, color.x * 255.99);
+}