0031668: Visualization - WebGL sample doesn't work on Emscripten 1.39

0031668: Visualization - WebGL sample doesn't work on Emscripten 1.39

OpenGl_Context now skips loading functions related to mapping buffer,
which are required by OpenGL ES 3.0 specs but not provided by WebGL 2.0.
Message_PrinterSystemLog does not use a broken emscripten_log() anymore, which corrupted UNICODE strings.

WasmOcctView::initWindow() - callbacks now set using EMSCRIPTEN_EVENT_TARGET_WINDOW
instead of 0 used by older Emscripten API.

Mouse callbacks now track canvas element and use
EmscriptenMouseEvent::targetX/targetY instead of ::canvasX/canvasY
as the latter was broken.

Added emscripten_set_main_loop() setup to shut up eglSwapInterval() error message.
Fixed missing \0 at the end of string converted by toUtf8Array().