textures become corrupted on game reload

[myk002]

to reproduce:

1. load a fort
2. quit to main menu without saving
3. load the fort again

The DFHack logo will become corrupted during the loading sequence.

@shevernitskiy

[myk002]

disregard -- this is likely due to the tests I was doing with cancel_job, which apparently is corrupting things.
no, this is an actual problem

[myk002]

it doesn't appear to be a refcount issue. increasing the refcount by 10 instead of 1 did not solve the issue.

[shevernitskiy]

it doesn't appear to be a refcount issue. increasing the refcount by 10 instead of 1 did not solve the issue.

if it something with refcount -> surface not exist -> nullref -> crash
our cached texpos (index of texture.raws vector) refering to not target texture