Encoding values with bits above the precision returns invalid results

[scaramallion]

Related to pydicom/pydicom#2260, when encoding if the pixel value uses bits above precision then the value is clamped to the maximum allowed by precision rather than ignoring the higher bits.

For example:

precision = 4
original = 0b0001_1011
after = 0b0000_1111  # should be 0b0000_1011

[scaramallion]

Unsigned integers: all bits above the precision should be set to 0
Signed integers: all bits above the precision should be set to 1 if the value < 0 else set to 0