Remove use of compile-time configuration macros
- BOOST_GIL_IO_TEST_ALLOW_READING_IMAGES
- BOOST_GIL_IO_TEST_ALLOW_WRITING_IMAGES
Narrow use of BOOST_GIL_IO_USE_BMP_TEST_SUITE_IMAGES macro.
Move test sources to format-specific directories.
Add FIXME and TODO comments about issues discovered while building the format-specific tests.
Add missing `#include`-s with standard library headers.
Apply minor corrections and tidying up.
Continues #459Closes#461