Add flipped GPU tiles example

[moufmouf]

Companion example for the TilemapGPULayer frame-edge sampling fix in phaserjs/phaser#7360 (linked from that PR).

Before (the bleeding can appear only briefly when loading the page):

(see the colored lines?)

After the fix in phaserjs/phaser#7360 :

TilemapGPULayer resolves a tile's pixels from the tileset in a fragment shader, so a flipped tile has to keep sampling inside its own frame even when a fragment lands exactly on a tile boundary.

This example fills a layer with one asymmetric tile flipped four ways — unflipped, flipX, flipY and both, one per quadrant — and surrounds that tile in the tileset with colours the map never uses: orange directly above it, green directly to its right. The camera creeps through sub-pixel offsets, which is when tile boundaries line up with fragment centres.

Nothing on screen should ever be orange or green. On stock 4.2.x, orange lines appear along the tile edges in the flipY quadrants and green ones in the flipX quadrants, flickering in and out as the camera moves. Swept over 512 sub-pixel offsets here: 6 orange rows and 8 green columns before the fix, none after.

One note on the example: the flips are set on the tiles rather than encoded in the map data on purpose. Tiled packs flips into the high bits of a GID, but ParseGID folds those three bits into rotation plus a single flipped flag, and ParseTileLayers assigns only tile.rotation and tile.flipX — so a map file can never produce a tile.flipY.