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/Renderer/RenderWindow.cpp

@@ -137,6 +137,7 @@ bool NzRenderWindow::OnCreate()
 {
 	m_parameters.doubleBuffered = true;
     m_parameters.window = GetHandle();
+
     m_context = new NzContext;
     if (!m_context->Create(m_parameters))
     {