BUG Raise exception for invalid input

Arrays of non-uint8 types were assumed to be in 0..1 range.

When this was not true and integer values were used, only the low-order
bits were used, resulting in a mangled image. Now, an explicit exception
is raised.

Closes issue matplotlib#2499

lib/matplotlib/cm.py

@@ -248,6 +248,8 @@ def to_rgba(self, x, alpha=None, bytes=False):
                     xx = x
                 else:
                     raise ValueError("third dimension must be 3 or 4")
+                if xx.dtype != np.uint8 and xx.max() > 1:
+                    raise ValueError("image must be either uint8 or in the 0..1 range")
                 if bytes and xx.dtype != np.uint8:
                     xx = (xx * 255).astype(np.uint8)
                 if not bytes and xx.dtype == np.uint8: