feat(vello_hybrid, vello_cpu): enable glifo/png feature to support bitmap emoji glyphs

[GoCoder7]

Summary

This PR enables the png feature of glifo by default in anyrender_vello_hybrid and anyrender_vello_cpu.

Motivation

Without glifo/png enabled, attempting to render bitmap emoji glyphs causes glifo to skip glyph rendering with GlyphSkipReason::PngSupportDisabled, resulting in invisible emoji characters in consumer applications (such as Blitz/Dioxus).

Changes

• Root Cargo.toml: enable features = ["png"] for workspace glifo = "0.4" dependency.
• crates/anyrender_vello_cpu/Cargo.toml: add png = ["glifo/png"] feature flag and expose it under default = ["png"].
• crates/anyrender_vello_hybrid/Cargo.toml: add png = ["glifo/png"] feature flag and expose it under default = ["png"].

[GoCoder7]

Closing this PR as enabling glifo/png causes a panic in vello_gpu (pixmap image sources are not supported by Vello GPU) when rendering bitmap emoji glyphs. Further architectural investigation with the GPU backend is needed before enabling this feature.