mesa/pack: refactor _mesa_pack_rgba_span_float()

Use autogenerated format pack functions and take advantage of some
macros to reduce source code, facilitating its maintenance.

Unfortunately, dstType == GL_UNSIGNED_SHORT cannot simplified like
the others, so keep it as it is.

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>

src/mesa/main/pack_tmp.h

@@ -26,6 +26,7 @@ FN_NAME(struct gl_context *ctx,
 	DST_TYPE *dst,
 	GLenum dstFormat,
 	SRC_TYPE rgba[][4],
+	GLfloat *luminance,
 	int n)
 {
    int i;