svga: round UBO constant buffer size up/down to multiple of 16 bytes

svga: round UBO constant buffer size up/down to multiple of 16 bytes

The svga3d device requires constant buffers to be a multiple of 16 bytes
in size.  OpenGL UBOs may not fit that restriction.  As a work-around,
round the size up if possible, else round down.

Note that this patch only effects UBO constant buffers (index 1 or higher),
not the 0th/default constant buffer.

Fixes the game Grim Fandango Remastered.  VMware bug 1510130.

Reviewed-by: Charmaine Lee <charmainel@vmware.com>
Reviewed-by: José Fonseca <jfonseca@vmware.com>