[fix] Properly account for MuPDF feeding us premultiplied alpha (#4807)

* Properly account for MuPDF feeding us premultiplied alpha

* Bump base to pickup necessary backend changes

Also includes a bunch of CMake refactoring
(https://github.com/koreader/koreader-base/pull/865
https://github.com/koreader/koreader-base/pull/867
https://github.com/koreader/koreader-base/pull/868)

base

@@ -1 +1 @@
-Subproject commit 6bd23e9dc0887211487804f6d7622889a4c2e042
+Subproject commit 9dd73333120a146746f1d2d363dacefeadf7627d

frontend/ui/widget/imagewidget.lua

@@ -359,7 +359,8 @@ function ImageWidget:paintTo(bb, x, y)
         -- Only actually try to alpha-blend if the image really has an alpha channel...
         local bbtype = self._bb:getType()
         if bbtype == Blitbuffer.TYPE_BB8A or bbtype == Blitbuffer.TYPE_BBRGB32 then
-            bb:alphablitFrom(self._bb, x, y, self._offset_x, self._offset_y, size.w, size.h)
+            -- NOTE: MuPDF feeds us premultiplied alpha (and we don't care w/ GifLib, as alpha is all or nothing).
+            bb:pmulalphablitFrom(self._bb, x, y, self._offset_x, self._offset_y, size.w, size.h)
         else
             bb:blitFrom(self._bb, x, y, self._offset_x, self._offset_y, size.w, size.h)
         end

frontend/ui/widget/textboxwidget.lua

@@ -431,7 +431,8 @@ function TextBoxWidget:_renderImage(start_row_idx)
         -- With alpha-blending if the image contains an alpha channel
         local bbtype = image.bb:getType()
         if bbtype == Blitbuffer.TYPE_BB8A or bbtype == Blitbuffer.TYPE_BBRGB32 then
-            self._bb:alphablitFrom(image.bb, self.width - image.width, 0)
+            -- NOTE: MuPDF feeds us premultiplied alpha (and we don't care w/ GifLib, as alpha is all or nothing).
+            self._bb:pmulalphablitFrom(image.bb, self.width - image.width, 0)
         else
             self._bb:blitFrom(image.bb, self.width - image.width, 0)
         end