Add SDL_ConvertSurface and fix SDL_SetAlpha

* Implement SDL_ConvertSurface (https://wiki.libsdl.org/SDL_ConvertSurface) works only for default emscripten pixelformat
* SDL_SetAlpha (http://www.libsdl.org/release/SDL-1.2.15/docs/html/sdlsetalpha.html) should ignore alpha channel if flag SDL_SRCALPHA is not set
Merge pull request #2871 from caiiiycuk/sdl_missed_fun

Add SDL_ConvertSurface and fix SDL_SetAlpha
cmake version note; fixes #2884
Add some WebGL2 fnuctions.
Add headers for OpenGL 4.2+.
Fix GL library behavior when deleting nonexisting objects via glDeleteProgram, glDeleteShader, glDeleteFramebuffers and glDeleteRenderbuffers. Note that OpenGL is inconsistent and glDeleteProgram and glDeleteShader do report errors when deleting invalid objects, the other functions don't.
Fix an issue with glDeleteProgram and glDeleteShader not cleaning up the GL.programs and GL.shaders array of already deleted objects.
Address review comments.
Add GLES3 headers.
update news
zlib: Fix compilation warning in example.c
test_zlib works on Node, so re-enable it.
emar can receive --em-config. Don't pass along to llvm-ar.

Fixes issue #2886.
Merge pull request #2885 from dvander/glTexStorage2D

Add some GLES3 functions for WebGL2.
Correct opaqueFrontBuffer to work correct with 8bpp surface
Merge pull request #2890 from caiiiycuk/opaqueFrontBuffer

Fix copyIndexedColorData to work correct with opaqueFrontBuffer flag
use ArrayBuffer.transfer if present, and log out memory growth times
optimize copyIndexedColorData by storing colors in 32-bit chunks; #2890
Merge pull request #2888 from waywardmonkeys/zlib_tests

Zlib tests
minor cleanups in minifyLocals