Added custom resolution size for main and game

[RaulGalvez288]

Partial fix: #1546

Implemented width and height parameters for main and game start modes.

Still have to add vsync and window modes.

[heinezen]

Hey thank you so much! I can already give you some feedback on the work you've done.

[RaulGalvez288]

I did want to ask if changing the engine.cpp and engine.h was alright for this issue? or if I should just use Window::render like #1553

[heinezen]

@RaulGalvez288 There is

openage/libopenage/renderer/window.h

Lines 27 to 38 in d09adb0

	struct window_settings {
	// Width of the window in pixels.
	size_t width = 1024;
	// Height of the window in pixels.
	size_t height = 768;
	// Graphics API to use in the window's renderer.
	graphics_api_t backend = graphics_api_t::DEFAULT;
	// If true, enable vsync.
	bool vsync = true;
	// If true, enable debug logging for the selected backend.
	bool debug = false;
	};

if you want to use that.