We were using the internal format to compute the size of the pixels
provided for texture image upload. They can differ in size from the
format/type combination.

Change-Id: I7f3cd535cdefefc3c7ec4695d35d181b43ae20a4
Reviewed-on: https://swiftshader-review.googlesource.com/16629Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>