Fill the alpha channel by default for vcl 32-bit bitmaps too.

Otherwise the alpha channel for bitmaps created directly is empty,
indicating a transparent bitmap (although we don't actually handle
transparency). This complements hardcoding of the alpha channel
in basebmp. VCL bitmaps can be copied bit-for-bit directly into
a basebmp bitmap, hence it's important to make sure we fill the
alpha channel in vcl too.

Change-Id: Icb2fa417db6625a6ffa6bd82eb5773ff75be5a3c
Kill gtktiledviewer's alpha channel hack.

basebmp and vcl now set the alpha channel appropriately, so no need
to do so in the viewer now.

However it would perhaps make more sense to just use RGB instead
of RGBA, seeing as the alpha channel is permanently set to be opaque.

Change-Id: I86ad758c6a8bee21b265730727a76605e5850c0c