Rename convert to process

More consistent with its "main" function's name

include/gfx/process.hpp

@@ -0,0 +1,14 @@
+/*
+ * This file is part of RGBDS.
+ *
+ * Copyright (c) 2022, Eldred Habert and RGBDS contributors.
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#ifndef RGBDS_GFX_CONVERT_HPP
+#define RGBDS_GFX_CONVERT_HPP
+
+void process();
+
+#endif /* RGBDS_GFX_CONVERT_HPP */