Fix glMultiDrawArraysIndirect not capturing client buffer

[Sturmlilie]

I'm not entirely sure why using Blob instead of OpaqueArray makes this work, but it does.
In case stride is zero, it means "the array is tightly packed" [1].

[1] https://www.khronos.org/registry/OpenGL/extensions/ARB/ARB_multi_draw_indirect.txt

[Sturmlilie]

I just realized that the command allows (and is primarily geared towards) usage with a buffer object bound, in which case this patch will likely copy from an invalid memory address. Will investigate how to differentiate between the two cases.

[jrfonseca]

It's not easy to do this sort of stuff (polymorphism) just from the spec, but it can be done.

See

apitrace/specs/gltypes.py

Line 305 in 15de874

def GLindexBuffer(countExpr, typeExpr):

for example, on how the index buffers are handled. They too can either be a memory pointer, or a buffer offset, depending on whether an index buffer is bound.