Decode PNG and JPEG textures with stb_image

[corepunch]

Summary

• Decode PNG and JPEG textures with vendored stb_image 2.30 (source/renderer/texture/r_image_stb.c) instead of libpng and libjpeg.
• Keep the previous upload behavior: PNG expands to RGBA, flips only for GL_TEXTURE_2D, and premultiplies when requested; JPEG uploads as RGB and is flipped. ASTC, DDS, and PVR are unchanged.
• Drop libjpeg and libpng from the engine pkg-config line, WebGL USE_LIB* flags, Xcode link flags, CI, and install docs. A missing libjpeg.pc was failing the whole pkg-config invocation and removing the Lua include path, which broke make cluster on macOS. Fonts still use FreeType.

Test plan

• make unite compiles every module and links liborca.so and orca with no libjpeg or libpng symbols in the renderer object.
• Vendored stb_image decodes a 2×2 PNG (top row red, second row green/blue).
• Run make cluster and confirm a screen that uses PNG or JPEG images still looks right, including premultiplied alpha and texture orientation.