[FEATURE] Improve the handling of retina tiles

[bramp]

What is the bug?

The TileLayer class has a retinaMode boolean, which when set triggers flutter_maps to request 4 tiles for every 1 and display them at half size, effectively doubling the resolution.

The urlTemplate supports a {r} placeholder, that is always replaced with a '@2x' asking the tile provider to return a tile at double the standard resolution (for example, requesting a 256x256 image, will have a 512x512 returned).

However, these things don't play well together. For example, if flutter_maps needs to fill a 256x256 square on the screen, one of the following three things will happen:

1. If {r} is in the urlTemplate, the @2x will always be added, regardless of the devices dpi (e.g even if the device only supports 1x, thus details from the image will be lost when displayed).

2. If retinaMode is set to true, then flutter_maps will fetch 4 squares (at one zoom lower) to make a 512x512 image for this space, and fill that in on the screen, thus simulating it. This only makes sense to do if the device supports dpi > 1x.

3. If the url contains {r} and retinaMode is set to true, we end up fetching 4 squares at 512x512 each (so total of 1024x1024), and jamming that into the 256x256 space.

So # 3 seems wasteful.

How can we reproduce it?

TileLayer(
   retinaMode: true,
   urlTemplate: 'https://..../{z}/{x}/{y}{r}',
)

Will attempt to both fetch 2x quality images, as well as make four requests for every 1 tile.

Do you have a potential solution?

After discussion on #1666, we think the correct behaviour should be:

TileLayer.retinaMode becomes a enum:

• off: don't do anything about retina mode
• on: if '{r}' is provided, fill it with '@2x', else simulate it
• auto (default): if the device's dpi > 1x then 'on', otherwise use 'off'

and for private testing

• simulate: simulate retina mode

Platforms

All

Severity

Minimum: Allows normal functioning

[JaffaKetchup]

One thing we haven't considered, which is I guess why this wasn't done before, is that we need the BuildContext to whether the device is high DPI. This has huge knock on effects, as the retina mode (simulation now) is used to change some other properties.