Enable asset load redirection

This patch allows to redirect the LoadAssetFileData function, so that other resource load mechanisms can be implemented. Specifically, assets can be loaded from resource data embedded in the executable. These resources can be produced with any resource compiler, including, for example https://github.com/vector-of-bool/cmrc.

This allows to create single-file-executables in Windows and Linux with all required resources embedded in the executable.

An alternative asset load implementation could look like this:

#include <cmrc/cmrc.hpp> // see https://github.com/vector-of-bool/cmrc
#include "hello_imgui/hello_imgui_assets.h"

CMRC_DECLARE(myAssets); // see https://github.com/vector-of-bool/cmrc 

auto resource_fs = cmrc::myAssets::get_filesystem();

[...]

HelloImGui::AssetFileData LoadAssetFileDataFromResource_Impl(const char *assetPath)
{
    HelloImGui::AssetFileData r;

    cmrc::file assetFile = resource_fs.open(assetPath);
    const char *begin = assetFile.cbegin();
    const char *end = assetFile.cend();
    r.dataSize = end - begin;
    if (r.dataSize)
    {
        r.data = malloc(r.dataSize);
        memcpy(r.data, begin, r.dataSize);
    }
    return r;
}

HelloImGui::AssetFileData LoadAssetFileDataFromResource(const char *assetPath)
{
    // assets starting with ":/" are ALWAYS loaded from the embedded resource file system
    if (strlen(assetPath)>2 && assetPath[0]==':' && assetPath[1] == '/') {
        assetPath++;
        if (!resource_fs.exists(assetPath))
        {
            std::stringstream msg;
            msg << "LoadAssetFileDataFromResource: cannot load " << assetPath << "\n";
            HIMG_ERROR(msg.str());
            return HelloImGui::AssetFileData();
        }
        else
        {
            return LoadAssetFileDataFromResource_Impl(assetPath);
        }
    }
    else
    {
        // other assets are searched on the embedded resource file system first.
        // If not found there, they are loaded from the standard assets folder
        if (resource_fs.exists(assetPath))
        {
            return LoadAssetFileDataFromResource_Impl(assetPath);
        }
        return HelloImGui::DefaultLoadAssetFileData(assetPath);
    }
}

int main(int, char **)
{
    HelloImGui::setLoadAssetFileDataFunction(LoadAssetFileDataFromResource);

    [...]
}

I'm not sure about the Doxygen comments, you may want to check them.

pthom / hello_imgui

Enable asset load redirection #106