feat: accept sub-byte bit depths for all color types

Previously, sub-byte bit depths (1-7 bits per sample) were only
accepted for Gray and Multiband color types. This extends the
validation to accept all color types (RGB, RGBA, CMYK, CMYKA, YCbCr)
at sub-byte depths.

The data format is unchanged: sub-byte samples remain packed MSB-first
in the output buffer, exactly as for grayscale. Predictors are still
rejected for sub-byte data.

Tested via libtiffpic flower-rgb-contig-02/04 and flower-rgb-planar-02/04.

src/decoder/image.rs

@@ -1030,7 +1030,12 @@ impl Image {
                 bit_depth: n,
                 num_samples: _,
             } if n == 8 || n == 16 || n == 32 || n == 64 => {}
-            ColorType::Gray(n)
+            ColorType::RGB(n)
+            | ColorType::RGBA(n)
+            | ColorType::CMYK(n)
+            | ColorType::CMYKA(n)
+            | ColorType::YCbCr(n)
+            | ColorType::Gray(n)
             | ColorType::Palette(n)
             | ColorType::Multiband {
                 bit_depth: n,