first, look at https://github.com/mrclnz/dwgthumb and the (probably old) bugs it fixes.
then new type 6 (== PNG)
```
overall size: 5764 [RL]
num_headers: 2 [RC]
	[0] Code: 1 [RC]
		Header data start: 487 [RL]
		Header data size: 80 [RL] (80)
	[1] Code: 6 [RC]
		Data start: 567 [RL]
		Size of unknown type 6: 5665 [RL]
No thumbnail bmp image in ../td/example_2018.dwg

overall size: 302219 [RL]
num_headers: 2 [RC]
	[0] Code: 1 [RC]
		Header data start: 3399 [RL]
		Header data size: 80 [RL] (80)
	[1] Code: 2 [RC]
		BMP data start: 3479 [RL]
		BMP size: 302120 [RL]
BMP offset: 119
ERROR: Preview overflow 119 + 302120 > 302223
No thumbnail bmp image in ../td/example_2007.dwg
```

* [x] add type 6 support (PNG), write with .png extension
* [x] add type 3 support (WMF), write with .wmf extension (no coverage!)
* [x] fix example_2007 overflow


better dwgbmp support
This is a GitHub Issue
repo:libredwg
owner:LibreDWG
Title : better dwgbmp support
Issue date:

--- start body ---
first, look at https://github.com/mrclnz/dwgthumb and the (probably old) bugs it fixes.
then new type 6 (== PNG)
```
overall size: 5764 [RL]
num_headers: 2 [RC]
	[0] Code: 1 [RC]
		Header data start: 487 [RL]
		Header data size: 80 [RL] (80)
	[1] Code: 6 [RC]
		Data start: 567 [RL]
		Size of unknown type 6: 5665 [RL]
No thumbnail bmp image in ../td/example_2018.dwg

overall size: 302219 [RL]
num_headers: 2 [RC]
	[0] Code: 1 [RC]
		Header data start: 3399 [RL]
		Header data size: 80 [RL] (80)
	[1] Code: 2 [RC]
		BMP data start: 3479 [RL]
		BMP size: 302120 [RL]
BMP offset: 119
ERROR: Preview overflow 119 + 302120 > 302223
No thumbnail bmp image in ../td/example_2007.dwg
```

* [x] add type 6 support (PNG), write with .png extension
* [x] add type 3 support (WMF), write with .wmf extension (no coverage!)
* [x] fix example_2007 overflow


--- end body ---
