Fix MSAA texture pointer not being freed (#3138)

Texture::destroy only freed m_ptr, not m_ptrMsaa. This caused
problems when texture IDs were reused, because a newly-
created texture would sometimes have m_ptrMsaa.

src/renderer_mtl.h

@@ -976,6 +976,7 @@ namespace bgfx { namespace mtl
 			if (0 == (m_flags & BGFX_SAMPLER_INTERNAL_SHARED))
 			{
 				MTL_RELEASE(m_ptr);
+				MTL_RELEASE(m_ptrMsaa);
 			}
 			MTL_RELEASE(m_ptrStencil);
 			for (uint32_t ii = 0; ii < m_numMips; ++ii)