Fail with pal2/pal4/rgb48 images

[AlexGuo1998]

I tried png-parser with pal2/pal4/rgb48 images:

pngparser -v -s test.png

... and they all fail. There are mainly 3 problems.

---

For all images, scanline_width is incorrect, so each scanline read is either too short or too long.

png-parser/pngparser/imagedata.py

Lines 232 to 234 in 4d75f17

	# line_width = pixel_count_in_line * bytes_count_by_pixel + the_filter_byte
	line_width = self.header.width * self.header.pixel_len
	logging.debug('%d bytes per scanline', line_width)

Guess it should be like:

# line_width_bit = pixel_count_per_line * components_per_pixel * bit_per_component
line_width_bit = self.header.width * self.header.pixel_len * self.header.bit_depth
# line_width = round_up(line_width_bit / 8) + the_filter_byte
line_width = (line_width_bit + 7) // 8 + 1

i.e. include bit_per_component(actually self.header.bit_depth) in calculation.

(I suggest renaming pixel_len to component_count, but it's also a little weird: paletted images can have more than 1 component, so maybe a better name?)

---

For pal2/pal4 images, it failed to convert index to pixel.

png-parser/pngparser/imagedata.py

Lines 332 to 333 in 4d75f17

	if self.palette:
	pixels = [self.palette[i] for i in row]

The correct way is:

if self.palette:
    bit_depth = self.header.bit_depth  # bit count per sample
    pixels = [self.palette[i] for i in BitArray(row, bit_depth)]

---

For rgb48 images, it failed to scale pixel value to 0~255 range when displaying.

png-parser/pngparser/imagedata.py

Lines 337 to 341 in 4d75f17

	for val in BitArray(row, bit_depth):
	if bit_depth != 8:
	# shrink value between 0 and 255
	val = val * (2**bit_depth - 1)
	px.append(val)

With rgb48, bit_depth is 16, i.e. val in range(0, 65536).
So val should be divided by 256, or be shifted 8 bits to the right:

if bit_depth > 8:
    # shrink value between 0 and 255
    val = val >> (bit_depth - 8)

---

I can craft a PR for these issues, but I'm not sure about the wording problem. Anyone has an idea?

[Hedroed]

Hi, thanks for all this fixes, I have not actually tested it on these types of images.
I'm OK with you about the pixel_len name. I suggest using the name used in the PNG specification (I don't know it).
I will be happy to accept your PR.