pnmshear: add page

pages/common/pnmshear.md

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+# pnmshear
+
+> Shear a PNM image.
+> More information: <https://netpbm.sourceforge.net/doc/pnmshear.html>.
+
+- Shear a PNM image by the specified angle:
+
+`pnmshear {{angle}} {{path/to/input.pnm}} > {{path/to/output.pnm}}`
+
+- Specify the color of the background in the sheared image:
+
+`pnmshear -background {{blue}} {{angle}} {{path/to/input.pnm}} > {{path/to/output.pnm}}`
+
+- Do not perform anti-aliasing:
+
+`pnmshear -noantialias {{angle}} {{path/to/input.pnm}} > {{path/to/output.pnm}}`