Rendering the combat screen intensively uses dynarrays (for blitting
lists storage). Any optimization of the dynarray functions can thus
bring a framerate improvement.

Currently, the dynarray functions internally use if-blocks to manage
sparse dynarrays, slowing down the execution time on non-sparse
dynarrays.

This patch extracts the sparse_dynarray specific code, and explicity
define sparse_dynarray_XXXX() functions.

Framerate improvement : 2.1GHz i7-3687U (HD 4000), level 0, max
zoom-out: from 54fps to 58fps (~9%)