Fix build errors with alloca/isinf/isnan/isfinite, using MinGW on Windows.
maketx --fullpixels
When given this command, maketx will consider the pixel data window of
the input image to be the full 0-1 texture range, ignoring any origin or
differenly specified full/display window (i.e. pretending that the image
is not, as its header may state, a crop of overscan image).  This is
primarily to counteract weird or buggy behavior in certain paint program
that inexplicably save images that look like crops even when they are
not intended to be.
maketx -- robustly remove "SHA-1=..." and "ConstantColor=..." from the
ImageDescription of the input image, without losing whatever else may be
in that metadata.