Add code to load and save pngs

Using libpng instead of the lowest-common-denominator
gdk-pixbuf loader. This will allow us to load >8bit data,
and apply gamma and color correction in the future.
For now, this still just provides RGBA8 data.

As a consequence, we are now linking against libpng.

subprojects/libpng.wrap

@@ -0,0 +1,12 @@
+[wrap-file]
+directory = libpng-1.6.37
+source_url = https://github.com/glennrp/libpng/archive/v1.6.37.tar.gz
+source_filename = libpng-1.6.37.tar.gz
+source_hash = ca74a0dace179a8422187671aee97dd3892b53e168627145271cad5b5ac81307
+patch_url = https://wrapdb.mesonbuild.com/v2/libpng_1.6.37-3/get_patch
+patch_filename = libpng-1.6.37-3-wrap.zip
+patch_hash = 6c9f32fd9150b3a96ab89be52af664e32207e10aa9f5fb9aa015989ee2dd7100
+
+[provide]
+libpng = libpng_dep
+