Add support for AC1032 (AutoCAD 2018) file format. #63
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Before the change object map section checksum was ignored. As a result, if we started to parse object map sections past its actual contents, we could read some garbage. Given that we the code computes checksum correctly, we now use it to verify if we are reading the right data. When we find invalid checksum, then we treat it as file error.
Add preliminary support for AC1032. Given that it is very similar to R2013 format (at least according to Open Design Specification for .dwg files Version' 5.4.1), we can try to read newer format as is without much additional work.
Sample AC1032 fomat files are a bit peculiar - Handles section size is larger than actual contents (looks like it is filled with padding). In the past we would base on section size, but now we are a bit smarter and stop reading object map sections upon finding empty section (as per ODA spec).
AC1032 file LayerControl object may refer to non-existant layers (non-existant handles). We cannot really do anything about it, so we just ignore such handles. Behaviour has been confirmed with ODA explorer.
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It turns out that existing dwg parser is almost good enough to handle newer files. This series add minimal modifications to enable reading (simple) AC1032 files. No AC1032 format-specific changes have been added yet, so the AC1032 format support is unfortunately still a bit broken, but simple drawings should be read correctly.