TRAC #11528

 * Initialise buffer to 0 if BufferData is called with data = NULL
   (special case: if not resizing, just leave the old data in place).
 * BufferSubData ignores calls with data = NULL
 * Replace Buffer::data_t with GLubyte.

Author:    Andrew Lewycky
Signed-off-by: Shannon Woods
Signed-off-by: Daniel Koch

git-svn-id: https://angleproject.googlecode.com/svn/trunk@59 736b8ea6-26fd-11df-bfd4-992fa37f6226