GameMaker: Studio mobile display setup (keeping aspect ratio without black bars)

room.gml

/* Paste this script inside your room creation code */

view_wview[0] = floor(global.viewWidth);
view_hview[0] = floor(global.viewHeight);
view_wport[0] = global.maxWidth;
view_hport[0] = global.maxHeight;
view_xview[0] = (global.idealWidth / 2) - (floor(global.viewWidth) / 2); //csantos: center-align view width
view_yview[0] = (global.idealHeight/ 2) - (floor(global.viewHeight)/ 2); //csantos: center-align view height 

display_set_gui_size(view_wview[0], view_hview[0]);
surface_resize(application_surface, view_wview[0], view_hview[0]);

scrSetupMobileDisplay.gml

/* This script must be executed once, ideally at game start */

var isPortrait = false;

//csantos: display variables (ideal width and height is the same as your room size)
global.idealWidth = 768;
global.idealHeight = 1024;
global.maxWidth = display_get_width();
global.maxHeight = display_get_height();
global.aspectRatio = display_get_width()/display_get_height();
global.viewWidth = 0;
global.viewHeight = 0;

//csantos: portrait
if(global.maxWidth < global.maxHeight) {
	isPortrait = true;
    global.viewWidth = min(global.idealWidth, global.maxWidth);
    global.viewHeight = global.viewWidth / global.aspectRatio;  
} 
//csantos: landscape
else {
    global.viewHeight = min(global.idealHeight, global.maxHeight);
    global.viewWidth = global.viewHeight * global.aspectRatio;  
}

var xView = (global.idealWidth / 2) - (floor(global.viewWidth) / 2);
var yView = (global.idealHeight / 2) - (floor(global.viewHeight) / 2);
									  
//csantos: adjusting for retina displays
if((isPortrait && xView > 0) || (!isPortrait && yView > 0)) {
    global.viewWidth = global.viewWidth*1.25;
    global.viewHeight = global.viewHeight*1.25;
}