vcl: Add support for backend-dependent blending of bitmaps (mask and alpha)

Change-Id: Iba64eb42965c86ca5655b9a105ef3f397e033ecf
vcl: Update current context before executing scale operation

Change-Id: I332c954259bdc7e34718449023f5aa82323987cc
vcl: Make sure the texture unit is the right one before binding

Change-Id: I392190a6927a6eb725c86bcf5278e3cd4491eb5c