Hillshade layers leak GPU memory

[ChrisLoer]

The hillshade layer reuses webgl textures as a performance optimization, and stores the textures for reuse in a cache in the painter object. The problem is that when we go to reuse existing textures, we look them up based on tile size:

mapbox-gl-js/src/render/draw_hillshade.js

Line 91 in 64ebdda

tile.demTexture = tile.demTexture || painter.getTileTexture(tile.tileSize);

But when we store them, we put them in a list based on the dimensions of their underlying texture:

mapbox-gl-js/src/render/painter.js

Lines 455 to 462 in 64ebdda

	saveTileTexture(texture: Texture) {
	const textures = this._tileTextures[texture.size[0]];
	if (!textures) {
	this._tileTextures[texture.size[0]] = [texture];
	} else {
	textures.push(texture);
	}
	}

These are not necessarily the same. For instance on a retina screen, 256px raster dem tiles will have a 514px texture (256x2 + 1px border on either side).

The result is that we cache an unbounded number of textures that we'll never re-use.

Thanks @grorg for reporting this in #4695 (comment)

cc @mollymerp