Remove implicit png feature, only have png-format

The optional `png` dependency creates an implicit `png` feature
which shows up when you do a `cargo add tiny-skia` but that won't
do anything when enabled. The right feature to enable is `png-format`
so we only need to have that one.

Cargo.toml

@@ -37,4 +37,4 @@ no-std-float = ["tiny-skia-path/no-std-float"]
 simd = []
 
 # Allows loading and saving `Pixmap` as PNG.
-png-format = ["std", "png"]
+png-format = ["std", "dep:png"]