Refactor InstantPageUI and related modules

submodules/Pdf/Sources/PDF.swift

@@ -0,0 +1,31 @@
+import Foundation
+import UIKit
+import Display
+
+public func generatePdfPreviewImage(url: URL, size: CGSize) -> UIImage? {
+    guard let data = try? Data(contentsOf: url, options: .mappedIfSafe) else { return nil }
+    return generatePdfPreviewImage(data: data, size: size)
+}
+
+public func generatePdfPreviewImage(data: Data, size: CGSize) -> UIImage? {
+    guard let provider = CGDataProvider(data: data as CFData) else { return nil }
+    guard let document = CGPDFDocument(provider) else { return nil }
+    guard let firstPage = document.page(at: 1) else { return nil }
+    
+    let context = DrawingContext(size: size)
+    context.withContext { c in
+        var pageRect = firstPage.getBoxRect(.mediaBox)
+        let pdfScale = 320.0 / pageRect.size.width
+        pageRect.size = CGSize(width: pageRect.size.width * pdfScale, height: pageRect.size.height * pdfScale)
+        pageRect.origin = CGPoint.zero
+        
+        c.setFillColor(UIColor.white.cgColor)
+        c.fill(pageRect)
+        
+        c.translateBy(x: 0.0, y: pageRect.size.height)
+        c.scaleBy(x: 1.0, y: -1.0)
+        c.concatenate(firstPage.getDrawingTransform(.mediaBox, rect: pageRect, rotate: 0, preserveAspectRatio: true))
+        c.drawPDFPage(firstPage)
+    }
+    return context.generateImage()
+}