Merge pull request #568 from marco-we/metal_destroy_program_invalidate_cache_fix

Added invalidation of renderpipeline state cache. In case the program…

src/renderer_mtl.mm

@@ -1453,6 +1453,8 @@ namespace bgfx { namespace mtl
 
 		m_processedUniforms = false;
 		m_numPredefined = 0;
+
+		m_renderPipelineStateCache.invalidate();
 	}
 
 	UniformType::Enum convertMtlType(MTLDataType _type)