GLsizei is historically defined as a 32-bit signed integer, which is
not allowed to be negative. But doing arithmetic with them can lead to
overflows producing undefined values, including negative ones.

Use size_t instead for these image data size calculations. It's defined
as an unsigned integer large enough to fit valid memory buffer sizes.
In practice that's 32-bit on 32-bit platforms, and 64-bit on 64-bit
ones, which is large enough to avoid overflow in the OutOfMemory test.

Bug: b/140421004
Change-Id: Id05dd64d470f579455fdc9925b0ab303ecaf37fb
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/36968Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>