Make tile data size a constant

[madmiraal]

Currently, gcc is raising an index overflow warning:

scene/2d/tile_map.cpp: In member function 'void TileMap::_set_tile_data(const PoolVector<int>&)':
scene/2d/tile_map.cpp:1528:22: error: writing 1 byte into a region of size 0 [-Werror=stringop-overflow=]
 1528 |             local[j] = ptr[j];
      |             ~~~~~~~~~^~~~~~~~
scene/2d/tile_map.cpp:1526:17: note: at offset 12 into destination object 'local' of size 12
 1526 |         uint8_t local[12];
      |                 ^~~~~

This is a false warning. The index will never be 12, because the loop limit will be less than 12 or 8:

RebelEngine/scene/2d/tile_map.cpp

Lines 1527 to 1529 in d310162

	for (int j = 0; j < ((format == FORMAT_2) ? 12 : 8); j++) {
	local[j] = ptr[j];
	}

However, to avoid confusion and prevent the limit being calculated every time, this PR makes the limit (the data size) a constant.