gpu.buffer.*'s "uri" output type let a client supply an arbitrary
stackWidth with no upper bound, used as the starting divisor in a loop
that decrements until it evenly divides the buffer's actual (small)
pixel count - a client sending a huge stackWidth (up to ~2 billion)
stalls the connection's handler thread for that many iterations.
Clamp it to the actual pixel count first.
gpu.buffer.texture's level parameter was forwarded as-is (u32) into
GPU_GetCurrentTexture(), which takes a plain int - a client-supplied
value whose u32->int conversion is negative skips backends' "level >=
mip count" bounds check (which only fires for level > 0), reaching
backend texture-copy code with a bogus mip index. Reject it upfront.
Henrik Rydgård
6929a0d9ae