Error improvement when call Assets.loadEverything before System.start

[RblSb]

Currently this mistake is complicated to debug, because:

• Loading callback if fired without problems
• Rendering without resources is fine
• g2.drawImage fails with:

Error: Cannot cast {
	g2canvas : null, 
	myWidth : 240, 
	myHeight : 160, 
	myFormat : 0, 
	renderTarget : false, 
	image : [object HTMLImageElement], 
	video : null
} to {
	__name__ : true, 
	init : <function>, 
	formatByteSize : <function>
}

And endless Error: End before you begin spam.

We can throw this problem at step 1 if do not ignore null in SystemImpl.gl here 10 years ago:

Kha/Backends/HTML5/kha/CanvasImage.hx

Lines 144 to 146 in 2462f98

	public function createTexture() {
	if (SystemImpl.gl == null)
	return;

Or maybe even earlier someway.