Add an internal ASSERT macro

This macro has some advantages over the standard library one:
 - It prints the place where the macro was triggered
 - It "references" the condition even in Release to avoid warnings
 - In release, if possible, it gives compiler hints

It is basically is stripped down version of the ASSERT macros I wrote
for the Daemon engine in src/common/Assert.h

This commit also removes the stray "backend" namespaces for common/
code.

src/backend/Texture.cpp

@@ -15,6 +15,7 @@
 #include "backend/Texture.h"
 
 #include "backend/Device.h"
+#include "common/Assert.h"
 
 namespace backend {
 
@@ -88,7 +89,7 @@ namespace backend {
     }
 
     void TextureBase::UpdateUsageInternal(nxt::TextureUsageBit usage) {
-        assert(IsTransitionPossible(usage));
+        ASSERT(IsTransitionPossible(usage));
         currentUsage = usage;
     }