handle numeric values of layer and level tags with math.floor before AttributeInteger

[etienneJr]

While calculating tiles for cartes.app, @laem and I noticed that numeric but non integer layer or level values make the calculation to crash, for example with this message :

Block 549/3727 lua runtime error 2:
maybe...Argument mismatch:string,number	 candidate is:
		std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >,int,
		std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >,int,char,

stack traceback:
	[C]: in function 'AttributeInteger'
	./resources/process-openmaptiles.lua:764: in function 'WritePOI'
	./resources/process-openmaptiles.lua:199: in function 'node_function'
table  `maybe...Argument mismatch:string,number	 candidate is:
		std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >,int,
		std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >,int,char,

stack traceback:
	[C]: in function 'AttributeInteger'
	./resources/process-openmaptiles.lua:764: in function 'WritePOI'
	./resources/process-openmaptiles.lua:199: in function 'node_function''  `maybe...Argument mismatch:string,number	 candidate is:
		std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >,int,
		std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >,int,char,

stack traceback:
	[C]: in function 'AttributeInteger'
	./resources/process-openmaptiles.lua:764: in function 'WritePOI'
	./resources/process-openmaptiles.lua:199: in function 'node_function'
stack traceback:'  
Lua error on node 676795020
Block 552/3727 Erreur de segmentation (core dumped)

It happens for example on these elements:

• https://www.openstreetmap.org/node/676795020 with level=-0.3
• https://www.openstreetmap.org/way/8351694 with layer=0.5

This PR fixes this bug using math.floor which is not perfect, but I did not find a round function in Lua ?

[systemed]

Thank you!