Use Texture2DArray instead of Texture3D for compressed pixel storage test.

Mesa drivers (rightfuly) complained that S3TC is not supported on 3D
textures, the packing had weird behavior on NVidia but it passed w/o
problems on AMD. Now should be okay on all three, yay!

src/Magnum/Test/PixelStorageGLTest.cpp

@@ -28,6 +28,7 @@
 #include "Magnum/Image.h"
 #include "Magnum/PixelFormat.h"
 #include "Magnum/Texture.h"
+#include "Magnum/TextureArray.h"
 #include "Magnum/TextureFormat.h"
 #include "Magnum/Test/AbstractOpenGLTester.h"
 
@@ -392,7 +393,7 @@ void PixelStorageGLTest::unpackCompressed3D() {
 
     CompressedImageView3D image{storage, CompressedPixelFormat::RGBAS3tcDxt3, {4, 4, 1}, CompressedData3D};
 
-    Texture3D texture;
+    Texture2DArray texture;
     texture.setStorage(1, TextureFormat::CompressedRGBAS3tcDxt3, {4, 4, 1})
         .setCompressedSubImage(0, {}, image);
 
@@ -413,7 +414,7 @@ void PixelStorageGLTest::packCompressed3D() {
 
     CompressedImageView3D actual{CompressedPixelFormat::RGBAS3tcDxt3, {4, 4, 1}, ActualCompressedData};
 
-    Texture3D texture;
+    Texture2DArray texture;
     texture.setStorage(1, TextureFormat::CompressedRGBAS3tcDxt3, {4, 4, 1})
         .setCompressedSubImage(0, {}, actual);