Update cheatsheets

ppmmix

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+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# ppmmix
+
+> Blend together two PPM images.
+> More information: <https://netpbm.sourceforge.net/doc/ppmmix.html>.
+
+- Blend the specified PPM images using fadefactor to control the weight of each image:
+
+`ppmmix {{fadefactor}} {{path/to/input_file1.ppm}} {{path/to/input_file2.ppm}} > {{path/to/output_file.ppm}}`