[BUG] I think ImageBufAlgo::make_texture() creates an unevictable tile in imagecache

[rasmusbonnedal]

Describe the bug We use ImageBufAlgo::make_texture(.., string_view filename, ..) to create a .tx file on the fly from a rather large PNG (30k x 15k 16-bit). When the conversion is done texture accesses to the newly created .tx are very slow. I observe that "Peak cache memory" is 2.6 GB after the call to make_texture and never goes down after that which leads to lots of redundant reads.

When we read the PNG to an ImageBuf and use the ImageBuf version of make_texture this problem does not occur.

To Reproduce I have attached a sample program which creates the PNG, performs make_texture on it and then samples the .tx file from 8 threads. When setting maketx_mode to USE_IMAGEBUF the texture sampling takes 1s on my computer. With USE_IMAGEFILE the sampling takes 60s.

With USE_IMAGEFILE:

    redundant reads: 529924 tiles, 12.1 GB
    Peak cache memory : 2.6 GB
  Image file statistics:
        opens   tiles    MB read   --redundant--   I/O time  res              File
      1    1        1     2688.6                       5.9s  30000x15662x3.u16  input.png  UNTILED
      2    1   537653    12601.2   (529924 12420.1)  8m 11.3s  30000x15662x3.u16  txfile.tx  MIP-COUNT[339938,197715,0,0,0,0,0,0,0,0,0,0,0,0,0]

  Tot:     2   537654    15289.8   (529924 12420.1)  8m 17.2s
  1 not tiled, 1 not MIP-mapped
  Broken or invalid files: 0

With USE_IMAGEBUF:

    redundant reads: 0 tiles, 0 B
    Peak cache memory : 181.3 MB
  Image file statistics:
        opens   tiles    MB read   --redundant--   I/O time  res              File
      1    1        0        0.0                       0.0s  30000x15662x3.u16  input.png  UNTILED
      2    1     7729      181.1                       1.0s  30000x15662x3.u16  txfile.tx  MIP-COUNT[4951,2778,0,0,0,0,0,0,0,0,0,0,0,0,0]

  Tot:     2     7729      181.1   (    0    0.0)      1.0s
  1 not tiled, 1 not MIP-mapped
  Broken or invalid files: 0

Expected behavior I expect the png data to vanish from the cache.

Platform information:

• OIIO branch/version: 2.3.12.0
• OS: Windows 10
• C++ compiler: Visual Studio 2019 16.11.8
• Any non-default build flags when you build OIIO: No
• libpng 1.6.37
• libtiff v4.3.0
• 9700K, 16 GB RAM oiio-repro.txt