update
Called once new bitmaps/textures are needed. Width and height are in pixels telling the size it will be drawn on screen and following them ensures that aspect ratio remains. But it's possible to return bitmap of any size though. You should use provided CurlPage for storing page information for requested page number. Index is a number between 0 and pageCount If provided, back index is the index to be used on the back side of page.
Parameters
page
page to be updated.
width
width of the page in pixels.
height
height of the page in pixels.
index
index of the page to be updated.
back
(optional) index of the back side of the page.