mask builders take prec, not integer

vips_gaussmat() and vips_logmat() used to default to float, but
vips_conv() defaulted to int ... so this failed:

	vips gaussmat m.v 2 0.1 --separable
	vips convsep k2.jpg x.jpg m.v

since we were using a small float mask with an integer convolution

This change makes everything int by default, with gaussmat having a new
param called --precision whose arg can be int or float.

The old @integer param is deprecated.