WIP: iris: Fix value of out-of-bounds accesses for vertex attributes

Having VERTEX_BUFFER_STATE.BufferSize greater than bound vertex buffer size allows shader to read uninitialized vertex attributes from BO instead of allowing hardware to return zeroes on out-of-bounds access.

OpenGL spec "6.4 Effects of Accessing Outside Buffer Bounds" says:

"Robust buffer access can be enabled by creating a context with robust access
enabled through the window system binding APIs. When enabled, any command
unable to generate a GL error as described above, such as buffer object accesses
from the active program, will not read or modify memory outside of the data
store of the buffer object and will not result in GL interruption or termination.
Out-of-bounds reads may return values from within the buffer object or zero
values."

Fixes three webgl tests:

 conformance/rendering/out-of-bounds-array-buffers.html
 conformance2/rendering/out-of-bounds-index-buffers-after-copying.html
 conformance2/rendering/element-index-uint.html

See #1996

Sent to CI and waiting for results.