OpenImageIO 2.1.19.0

Release 2.1.19 (1 Sep 2020) -- compared to 2.1.18

• DPX: Add support for reading DPX files from IOProxy (such as from a memory
  buffer). #2659 #2665
• PNG: New output option "png:filter" allows control of the PNG filter
  options. #2650
• Python: Fix binding of ImageSpec.erase_attribute. #2654
• Python: Fix missing ImageInput.read_image(). #2677
• Windows: Improve Strutil::get_rest_arguments() handling of long path
  syntax ("\\?\" style). #2661
• MinGW: Fix a variety of compiler warnings on this platform. #2657
• Fix build on Elbrus 2000 architecture. #2671