opengl32/tests: Only use functions if they are implemented.

dlls/opengl32/tests/opengl.c

@@ -236,6 +236,12 @@ static void test_colorbits(HDC hdc)
     int res;
     int iPixelFormat = 0;
 
+    if (!pwglChoosePixelFormatARB)
+    {
+        skip("wglChoosePixelFormatARB is not available\n");
+        return;
+    }
+
     /* We need a pixel format with at least one bit of alpha */
     res = pwglChoosePixelFormatARB(hdc, iAttribs, NULL, 1, &iPixelFormat, &nFormats);
     if(res == FALSE || nFormats == 0)
@@ -264,6 +270,12 @@ static void test_gdi_dbuf(HDC hdc)
     int iPixelFormat;
     int res;
 
+    if (!pwglGetPixelFormatAttribivARB)
+    {
+        skip("wglGetPixelFormatAttribivARB is not available\n");
+        return;
+    }
+
     nFormats = DescribePixelFormat(hdc, 0, 0, NULL);
     for(iPixelFormat = 1;iPixelFormat <= nFormats;iPixelFormat++)
     {
@@ -383,6 +395,13 @@ START_TEST(opengl)
         test_colorbits(hdc);
         test_gdi_dbuf(hdc);
 
+        if (!pwglGetExtensionsStringARB)
+        {
+            skip("wglGetExtensionsStringARB is not available\n");
+            DestroyWindow(hwnd);
+            return;
+        }
+
         wgl_extensions = pwglGetExtensionsStringARB(hdc);
         if(wgl_extensions == NULL) skip("Skipping opengl32 tests because this OpenGL implementation doesn't support WGL extensions!\n");