add file format documentation

doc/file_data_spec.txt

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+HDR
+entry 
+{
+  16B char[16] section name
+  12B zero
+  4B base offset
+}
+
+DAT
+section 
+{
+  4B offset
+  4B size
+  xB file data, mod4 padded
+  entry 
+  {
+    4B int: file offset
+    4B int: file size, if directory then sum of all its file sizes
+    4B int: 0xFFFFFFFF padding 
+    2B int: type flag:
+               %10000000 - folder
+               %00000010 - file
+               %00000000 - file
+               %10000010 - subfolder?
+    2B int: if directory then sum of all subentry sizes plus this entry size (subentry count * 32 + 32)
+    16B char: filename (replace last "_" with "." if any)
+  }  
+}
+
+
+
+

doc/file_hmp_spec.txt

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+15B zeros
+1B ? allways 0x3f (63)
+4B ?
+4B ? allways 0x0000003f (63)
+2B tile count
+2b ?
+4B offset to tiles
+4B offset to some data?
+2B width in BLK
+2B height in BLK
+BLK 
+{
+  width * height * 2B tile indices
+}
+xB ?
+tiles 
+{
+   2b texmap idx (from texture index file)
+   1b ?
+   1b lo - minimum height in tile (probably for terrain LOD?)
+   1b hi - maximum height in tile
+   25B - 5x5 heights
+}
+2B 0x0000
+   

doc/file_image_spec.txt

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+4B data size DS
+4B offset to pixels
+32B zeros
+4B description string offset DSO
+attributes {
+  2B width - must be increased to multiple of 2 if odd
+  2B height
+  1B 0x01
+  1B bits per pixel:
+      -0: 4bit
+      -1: 8bit
+  1B subtype:
+     -3 RGBA
+     -4 greyscale
+     -5 ? ov_rdir
+  1B 0, 0x80/128
+}
+4B zeros
+4B ?
+xB pixels
+(DSO - DS)B description string (zero term.)
+
+if there's more data in file, repeat
+
+
+subtype:
+0 - palette 16x3B RGB, 4bit per pixel
+1 - ?256x4B palette RGBA, 8bit per pixel?
+2 - ?16bit RGB?
+3 - RGBA 32bit
+4 - grayscale, 4bit per pixel if bpp = 0, else grayscale 8 bit
+5 - grayscale, 16bit per pixel