NzImage now handles mipmaps

Added NzRenderer::SetClearColor(const Color&)
Added describe comment for pixel formats
Fixed NzPixelFormat conversion for RGBA4 and RGB5A1 types
Fixed NzRenderer::Clear prototype
Renamed NzLock to NzLockGuard
ResourceLoader's loaders are now tested from newest to oldest

src/Nazara/Utility/Loaders/STB.cpp

@@ -120,6 +120,7 @@ namespace
 		}
 
 		resource->Update(ptr);
+
 		stbi_image_free(ptr);
 
 		if (stbiFormat == STBI_default && parameters.loadFormat != nzPixelFormat_Undefined)