Fix false-positive AMD/NVidia driver detection on OSX.

Apple has its own drivers but still names the vendor the same way as if it
were the Windows/Linux binary drivers. Weird and/or sneaky.

src/Magnum/Context.h

@@ -212,7 +212,7 @@ class MAGNUM_EXPORT Context {
             #endif
 
             #ifndef MAGNUM_TARGET_WEBGL
-            /** Binary NVidia drivers */
+            /** Binary NVidia drivers on Windows and Linux */
             NVidia = 1 << 2,
             #endif
 

src/Magnum/Implementation/driverSpecific.cpp

@@ -115,6 +115,8 @@ auto Context::detectedDriver() -> DetectedDrivers {
 
     const std::string vendor = vendorString();
 
+    /* Apple has its own drivers */
+    #ifndef CORRADE_TARGET_APPLE
     #ifndef MAGNUM_TARGET_GLES
     /* AMD binary desktop drivers */
     if(vendor.find("ATI Technologies Inc.") != std::string::npos)
@@ -131,6 +133,7 @@ auto Context::detectedDriver() -> DetectedDrivers {
     if(vendor.find("NVIDIA Corporation") != std::string::npos)
         return *_detectedDrivers |= DetectedDriver::NVidia;
     #endif
+    #endif
 
     /** @todo there is also D3D9/D3D11 distinction on webglreport.com, is it useful? */
     #ifdef MAGNUM_TARGET_GLES