move tiles srcrect dimention to config.vars

[rexim]

introduced in #117

That may require add a new type to the config file.

Might be a good opportunity to simplify adding new types to the system.

[kolumb]

Currently srcrect dimention is defined like this: https://github.com/tsoding/something/blob/17f33281e1f074df8743d507219477721e1d05de/src/something_item.cpp#L44 And you would like it to be defined something like this? https://github.com/tsoding/something/blob/17f33281e1f074df8743d507219477721e1d05de/assets/config.vars#L83-L86

ITEM_HEALTH_TEXTURE_SRCRECT : rect = {0, 0, 64, 64}