Allow disabling image loading and saving functionalities

[takase1121]

I am testing out PlutoSVG to render OT-SVG fonts in Lite XL. I'm impressed by the results (it did work) but I've noticed a 400kb increase in the binary size, in -O3 stripped. I then noticed that plutovg is vendoring stb_image to load surfaces from images and write surfaces to files. I am only planning to use Pluto(S)VG to render graphics to memory, so I didn't need to stb_image and stb_image_write to read and write images.

This PR includes the PLUTOVG_USE_STB_IMAGE (CMake) and use_stb_image (meson) options to enable / disable image support. This would also indirectly allow the library to support other image libraries in the future, like how nanovg/fontstash.h supports FreeType and stbtt for rendering text. The following screenshot shows 300kb being saved by just omitting stb_image.h and stb_image_write.h.

stb_image included	stb_image not included

[sammycage]

Thank you for your contribution! I understand your concern about the increase in binary size. It's fantastic to hear that PlutoSVG is performing well for rendering OT-SVG fonts in Lite XL!

Image loading is crucial for PlutoSVG to fully adhere to the OTF-SVG specifications, as it must support PNG and JPEG formats for accurate rendering. Your suggestion to provide options for enabling or disabling image support is a valuable insight.