Proper detection for AMD driver bug workaround.

Not sure where I got that.

src/Magnum/Implementation/setupDriverWorkarounds.cpp

@@ -35,8 +35,8 @@ void Context::setupDriverWorkarounds() {
 
     #ifndef MAGNUM_TARGET_GLES
     /* This extension causes crash in GLSL compiler on AMD linux drivers 13.251 */
-    const std::string renderer = rendererString();
-    if(renderer.find("Advanced Micro Devices") != std::string::npos)
+    const std::string vendor = vendorString();
+    if(vendor.find("ATI Technologies Inc.") != std::string::npos)
         _setRequiredVersion(GL::ARB::explicit_uniform_location, None);
     #endif