* added write as pbm
* add tests for pbm
* imgcodecs: PBM support
- drop additional PBM parameters
- write: fix P1/P4 mode (no maxval 255 value after width/height)
- write: invert values for P1/P4
- write: P1: compact ASCII mode (no spaces)
- simplify pbm test
- drop .pxm extension (http://netpbm.sourceforge.net/doc/ doesn't know such extension)
Root cause: when calling imread to read the pgm file, when returning with success reading file, the destructor will free the resource which used by memcpy, but in the code, the resource calculation was not correct, therefore, it will cause memory corruption during resource free.
