shaped-texture: Use a double when calculating clip

For x defined below, x == -INT32_MAX assuming that the arithmetic
expression actually uses the fpu.

float f = 1.0f;
int32_t x = INT32_MAX * f;

This would result in the calculated clip width/height to be -INT_MAX
if the damage width/height is INT_MAX. To solve this, use a double
variable instead.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>

https://bugzilla.gnome.org/show_bug.cgi?id=705502