When PlayResX or PlayResY are defined as zero, tab crashes.

[joshuabrown-ellation]

To reproduce:

• set the advanced substation subtitle (.ass) PlayResX or PlayResY property to 0 in Aegisub and save the file. Or you can edit the .ass file directly, i.e. PlayResX: 0.
• use libjass to render that subtitle file via URL or by directly using the text of the .ass file.

Identification of problem:
Libjass uses property PlayResX and PlayResY for scaling of the rendering of effects in CSS, for example _textShadow. The way they are defined is by element width / PlayResX and element height / PlayResY, creating the possibility of a divide by zero condition. Since Javascript defines n / 0 as Infinity, and Libjass tries to setup a for() loop with that number as the break condition, we will run out of memory abruptly.

Proposed solution:
Line 346 of renderer.ts:
if (this.ass.properties.resolutionX === 0 || this.ass.properties.resolutionY === 0) { this._scaleX = 0; this._scaleY = 0; } else { this._scaleX = width / this.ass.properties.resolutionX; this._scaleY = height / this.ass.properties.resolutionY; }

[Arnavion]

Relevant PlayRes* fixups in libass are at https://github.com/libass/libass/blob/6092e276de387133de4dfb17843a5d8d0b8de3f0/libass/ass.c#L1333

Pseudocode:

switch (x, y) {
	(> 0, > 0) => (x, y),
	(<= 0, <= 0) => (384, 288),
	(1280, <= 0) => (1280, 1024),
	(<= 0, 1024) => (1280, 1024),
	(*, <= 0) => (x, x * 3 / 4),
	(<= 0, *) => (y * 4 / 3, y),
}

libjass should do the same.