Unity webgl!

Thread here: https://2dimensions.slack.com/archives/CHMAP278R/p1709934972246289

![image](https://github.com/rive-app/rive/assets/454182/cc72ee80-4506-4e45-a65a-d994cae0a1a8)

Uses PLS when draft extensions are enabled in Chrome!
![image](https://github.com/rive-app/rive/assets/454182/fadb112f-1bff-457f-98a1-6fe524224398)

Details on how it works and how to patch Unity's emscripten here (part of this PR which will push downstream):
https://github.com/rive-app/rive/blob/unity_webgl/packages/runtime_unity/public/WEBGL.md

Diffs=
bcf6451f4 Unity webgl! (#6816)

Co-authored-by: Luigi Rosso <luigi-rosso@users.noreply.github.com>

.rive_head

@@ -1 +1 @@
-09feaccb0c5bb3a2f88c23aca6c669d79eee4428
+bcf6451f442d7d55ed423982b58204e93c1d8791

dependencies/premake5_libpng.lua

@@ -3,6 +3,9 @@ local dependency = require('dependency')
 libpng = dependency.github('glennrp/libpng', 'libpng16')
 zlib = dependency.github('madler/zlib', '04f42ceca40f73e2978b50e93806c2a18c1281fc')
 
+includedirs({ './' })
+forceincludes({ 'rive_png_renames.h' })
+
 project('libpng')
 do
     kind('StaticLib')
@@ -11,7 +14,7 @@ do
     targetdir('%{cfg.system}/cache/bin/%{cfg.buildcfg}/')
     objdir('%{cfg.system}/cache/obj/%{cfg.buildcfg}/')
     os.copyfile(libpng .. '/scripts/pnglibconf.h.prebuilt', libpng .. '/pnglibconf.h')
-    includedirs({ './', libpng, zlib })
+    includedirs({ libpng, zlib })
     files({
         libpng .. '/png.c',
         libpng .. '/pngerror.c',

dependencies/premake5_libpng_v2.lua

@@ -4,11 +4,14 @@ local dependency = require('dependency')
 libpng = dependency.github('glennrp/libpng', 'libpng16')
 zlib = dependency.github('madler/zlib', '04f42ceca40f73e2978b50e93806c2a18c1281fc')
 
+includedirs({ './' })
+forceincludes({ 'rive_png_renames.h' })
+
 project('libpng')
 do
     kind('StaticLib')
     os.copyfile(libpng .. '/scripts/pnglibconf.h.prebuilt', libpng .. '/pnglibconf.h')
-    includedirs({ './', libpng, zlib })
+    includedirs({ libpng, zlib })
     files({
         libpng .. '/png.c',
         libpng .. '/pngerror.c',