Clear bsp_data from bsp_info after it has been used

[Shivansps]

After taking a look at were bsp_data is used, i noticed that only submodel_get_num_polys() used it after it had completed model_load(), and this function was called only from freespace2.cpp when the -pofspew was used.

So i changed it to save the result of submodel_get_num_polys_sub() that submodel_get_num_polys() used when the -pofspew cmdline is used. Then i reseted the buffer.

Then i noticed -pofspew was causing an abort because submodel_get_num_polys_sub() had an infinite recursion bug when one of the list had a value of 0 and called submodel_get_num_polys_sub() again.
I confirmed this by running -pofspew on the current nightly and it crashes before exporting all models on BP 3.3.3 dataset, so this was probably broken for like, forever. I added a check to avoid this and how it works again.

As for the results of clearing the bsp_data cache:
On Icarus (with a dgpu)

Before:

After:

Im not noticing any problems with this change, but ill recommend someone who knows more about the model system (and virtual pof) might want to check it out first.

P.D. i moved model_get_textures_used() declaration from models.h to modelinterp.h i just added because it seems cleaner since i was adding a new header.

[Goober5000]

Looks good overall; one thing

[Goober5000]

This unfortunately has the side effect of leaving a collision tree with dangling data. So model_remove_bsp_collision_tree should be updated to set the freed pointers to nullptr and the counts to 0.

[Shivansps]

right, then ill remove the if and the continue and move the nullprt check on before calling submodel_get_num_polys_sub()