FixedWidthVaryingHeightLayout constructor
      
      FixedWidthVaryingHeightLayout(
    
- int columns,
- double columnWidth,
- double spacing,
- GiphySource source,
Creates a new sliver grid layout
Implementation
FixedWidthVaryingHeightLayout(
    this.columns, this.columnWidth, this.spacing, this.source)
    : _columnOffsets = List.generate(columns, (index) => 0.0) {
  for (var i = 0; i < source.cacheCount; i++) {
    _calculateGeometry(i);
  }
}