add: support sdl3 + opengl 3.3 and opengles 3.0

[vsaint1]

• Support for both desktop (OpenGL 3.3) and mobile/web (OpenGL ES 3.0)
• Example main menu with settings (resolution, difficulty, volume, etc..)

Here are some examples tested on different platforms:

[vsaint1]

Forgot to add

[oreo639]

This doesn't work. The type of evt->text.text changed between SDL2 and SDL3. With SDL2, this is an array of size SDL_TEXTINPUTEVENT_TEXT_SIZE, so while it isn't correct, it wouldn't crash. With SDL3, this is now a pointer to a UTF-8 string, so this will now crash reading OOB.

The OOB issue can be fixed by using strncpy(glyph, evt->text.text, NK_UTF_SIZE) instead of memcpy(), but that doesn't fix the issue that evt->text.text can have more than one character.

For the latter issue, smth like the following should work afaict:

nk_rune unicode;
int glyph_len;
int byte_len;
const char *text = evt->text.text;
glyph_len = byte_len = nk_utf_decode(text, &unicode, 4);
while (unicode != '\0' && glyph_len) {
	nk_input_unicode(ctx, unicode);
 	glyph_len = nk_utf_decode(text+byte_len, &unicode, 4);
    byte_len += glyph_len;
}

(This will be able to store up to NK_INPUT_MAX from the SDL3 text input buffer)

[vsaint1]

this is correct, didn't crashed but i need to make some minor changes, thanks for showing this!!!