Added documentation for Emscripten

EMSCRIPTEN.md

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+# Using libprojectM in Emscripten
+
+projectM supports OpenGL ES rendering, and can be compiled into WebAssembly for use in browsers. WebGL is similar to
+OpenGL ES, but not identical, so a few additional considerations apply to get projectM running with Emscripten.
+
+## Additional Build Settings
+
+A few additional build settings will be required when building an Emscripten wrapper. Pass these flags/parameterrs to
+the Emscripten linker:
+
+- `-sUSE_SDL=2`: It is highly recommended to use Emscripten's built-in SDL2 port to set up the rendering context. This
+  flag will link the appropriate library.
+- `-sMIN_WEBGL_VERSION=2 -sMAX_WEBGL_VERSION=2`: Forces the use of WebGL 2, which is required for OpenGL ES 3 emulation.
+- `-sFULL_ES2=1 -sFULL_ES3=1`: Enables full emulation support for both OpenGL ES 2.0 and 3.0 variants.
+- `-sALLOW_MEMORY_GROWTH=1`: Allows allocating additional memory if necessary. This may be required to load additional
+  textures etc. in projectM.
+
+## Initializing Emscripten's OpenGL Context
+
+In addition to the above linker flags, some additional initialization steps must be performed to set up the OpenGL
+rendering context for projectM. Specifically, the `OES_texture_float` WenGL extension must be loaded explicitly to
+support the required texture format for the motion vector grid. The following code template can be used to set up a
+proper SDL2/WebGL context for projectM:
+
+```c
+
+#include <emscripten.h>
+#include <emscripten/html5_webgl.h>
+
+#include <GL/gl.h>
+
+#include <SDL.h>
+
+int main(void)
+{
+    // Init SDL's video and audio subsystems
+    SDL_Init(SDL_INIT_VIDEO | SDL_INIT_AUDIO);
+
+    // Create the SDL window (will be tied to the Emscripten HTML5 canvas)
+    SDL_window* window = NULL;
+    SDL_renderer* renderer = NULL;
+    SDL_CreateWindowAndRenderer(1024, 768, SDL_WINDOW_OPENGL, &window, &renderer);
+    if (window == NULL || renderer == NULL)
+    {
+        fprintf(stderr, "Failed to create SDL renderer: %s\n", SDL_GetError());
+        return 1;
+    }
+
+    // Enable floating-point texture support for motion vector grid.
+    auto webGlContext = emscripten_webgl_get_current_context();
+    emscripten_webgl_enable_extension(webGlContext, "OES_texture_float");
+
+    // Initialize projectM and put all other stuff below.
+    
+    return 0;
+}
+
+```