Fixed build warnings

src/render/metal/SDL_render_metal.m

@@ -627,7 +627,7 @@ static int METAL_CreateTexture(SDL_Renderer *renderer, SDL_Texture *texture, SDL
         METAL_RenderData *data = (__bridge METAL_RenderData *)renderer->driverdata;
         MTLPixelFormat pixfmt;
         MTLTextureDescriptor *mtltexdesc;
-        id<MTLTexture> mtltexture, mtltextureUv;
+        id<MTLTexture> mtltexture = nil, mtltextureUv = nil;
         BOOL yuv = FALSE;
         BOOL nv12 = FALSE;
         METAL_TextureData *texturedata;
@@ -696,7 +696,9 @@ static int METAL_CreateTexture(SDL_Renderer *renderer, SDL_Texture *texture, SDL
         }
 
         if (surface) {
-            mtltexture = [data.mtldevice newTextureWithDescriptor:mtltexdesc iosurface:surface plane:0];
+            if (@available(iOS 11.0, *)) {
+                mtltexture = [data.mtldevice newTextureWithDescriptor:mtltexdesc iosurface:surface plane:0];
+            }
         } else {
             mtltexture = [data.mtldevice newTextureWithDescriptor:mtltexdesc];
         }
@@ -727,7 +729,9 @@ static int METAL_CreateTexture(SDL_Renderer *renderer, SDL_Texture *texture, SDL
 
         if (yuv || nv12) {
             if (surface) {
-                mtltextureUv = [data.mtldevice newTextureWithDescriptor:mtltexdesc iosurface:surface plane:1];
+                if (@available(iOS 11.0, *)) {
+                    mtltextureUv = [data.mtldevice newTextureWithDescriptor:mtltexdesc iosurface:surface plane:1];
+                }
             } else {
                 mtltextureUv = [data.mtldevice newTextureWithDescriptor:mtltexdesc];
             }