Revert "vbo: Don't discount stride == 0 for testing all varyings in VBOs."

Revert "vbo: Don't discount stride == 0 for testing all varyings in VBOs."

This reverts commit d631c19db47181129811080bfa772b210d762d4d.

The commit was broken, and ended up returning false all the time
because nobody in the world binds every single possible vertex array.
On further reflection, we don't want to discount stride == 0: This
function is just used for deciding to calculate whether to compute the
bonuds on the index, and there's no sense in computing index bounds
when stride == 0.

For the separate question of "how much data do I upload for this
vertex element?", the i965 driver was fixed to upload the data.

Fixes a regression of about 2x in 3DMMES, and most importantly, makes
Hammerfight playable.