Add Emscripten sample builds

[RobLoach]

This adds definitions for building for the web. You will need emcc installed. Also need to run a web server that supports .wasm file requests.

make platform=web

Caveat

sample-effect's doesn't work since load_image() loads an image using stbi_load(). File system loading has to be switched to use sokol_fetch.h instead of through straight up file system.

[edubart]

Seems to work for me with emcc, thanks, merged!

I have also improved it to make sample effects to work and added to github pages. So you can view the demos on github in the following links:

Primitives sample - https://edubart.github.io/sokol_gp/sample-primitives.html Blending modes sample - https://edubart.github.io/sokol_gp/sample-blend.html Frame buffer sample - https://edubart.github.io/sokol_gp/sample-framebuffer.html Rectangle sample - https://edubart.github.io/sokol_gp/sample-rectangle.html Effect sample - https://edubart.github.io/sokol_gp/sample-effect.html SDF sample - https://edubart.github.io/sokol_gp/sample-sdf.html