Fixed crash if initialization of EGL failed but was tried again later.

The internal function SDL_EGL_LoadLibrary() did not delete and remove a mostly
uninitialized data structure if loading the library first failed. A later try to
use EGL then skipped initialization and assumed it was previously successful
because the data structure now already existed. This led to at least one crash
in the internal function SDL_EGL_ChooseConfig() because a NULL pointer was
dereferenced to make a call to eglBindAPI().

docs/README-emscripten.md

@@ -0,0 +1,33 @@
+Emscripten
+================================================================================
+
+Build:
+
+    $ emconfigure ./configure --host=asmjs-unknown-emscripten --disable-assembly --disable-threads --enable-cpuinfo=false CFLAGS="-O2"
+    $ emmake make
+
+Or with cmake:
+
+    $ emconfigure cmake ..
+    $ make
+
+To build one of the tests:
+
+    $ cd test/
+    $ emcc -O2 --js-opts 0 -g4 testdraw2.c -I../include ../build/.libs/libSDL2.a ../build/libSDL2_test.a -o a.html
+
+Uses GLES2 renderer or software
+
+tests: https://dl.dropboxusercontent.com/u/17360362/SDL2-em/index.html
+
+Some other SDL2 libraries can be easily built (assuming SDL2 is installed somewhere):
+
+SDL_mixer (http://www.libsdl.org/projects/SDL_mixer/)
+
+    $ EMCONFIGURE_JS=1 emconfigure ../configure
+    build as usual...
+
+SDL_gfx (http://cms.ferzkopp.net/index.php/software/13-sdl-gfx):
+
+    $ EMCONFIGURE_JS=1 emconfigure ../configure --disable-mmx
+    build as usual...