ST_TileEnvelope backport

ST_TileEnvelope.sql

--  ported from https://github.com/pramsey/minimal-mvt/blob/58f3e695a305f42024dcf0ba395590bf39b0b573/minimal-mvt.py#L63-L81
CREATE OR REPLACE FUNCTION ST_TileEnvelope(tileZoom integer, tileX integer, tileY integer) RETURNS geometry AS $$
-- Width of world in EPSG:3857
DECLARE	worldMercMax float = 20037508.3427892;
DECLARE worldMercMin float = -1 * worldMercMax;
DECLARE worldMercSize float = worldMercMax - worldMercMin;
-- Width in tiles
DECLARE worldTileSize float = power(2, tileZoom);
-- Tile width in EPSG:3857
DECLARE tileMercSize float = worldMercSize / worldTileSize;
-- Calculate geographic bounds from tile coordinates
-- XYZ tile coordinates are in "image space" so origin is
-- top-left, not bottom right
DECLARE xmin float = worldMercMin + tileMercSize * tileX;
DECLARE xmax float = worldMercMin + tileMercSize * (tileX + 1);
DECLARE ymin float = worldMercMax - tileMercSize * (tileY + 1);
DECLARE ymax float = worldMercMax - tileMercSize * (tileY);
BEGIN
	RETURN ST_MakeEnvelope(xmin, ymin, xmax, ymax, 3857);
END;
$$ LANGUAGE 'plpgsql' IMMUTABLE;


-- OR... copied from https://github.com/CrunchyData/pg_tileserv
CREATE OR REPLACE
FUNCTION ST_TileEnvelope(z integer, x integer, y integer)
RETURNS geometry
AS $$
  DECLARE
    size float8;
    zp integer = pow(2, z);
    gx float8;
    gy float8;
  BEGIN
    IF y >= zp OR y < 0 OR x >= zp OR x < 0 THEN
        RAISE EXCEPTION 'invalid tile coordinate (%, %, %)', z, x, y;
    END IF;
    size := 40075016.6855784 / zp;
    gx := (size * x) - (40075016.6855784/2);
    gy := (40075016.6855784/2) - (size * y);
    RETURN ST_SetSRID(ST_MakeEnvelope(gx, gy, gx + size, gy - size), 3857);
  END;
$$
LANGUAGE 'plpgsql'
IMMUTABLE
STRICT
PARALLEL SAFE;