PVRTC: incorrect alpha decoding

[attilaz]

pipes.zip
This image clearly shows the artifacts.

One problem seems to be the incorrect punchthrough flag. According to the specs (table 173) it is at bit 32.
https://www.khronos.org/registry/DataFormat/specs/1.3/dataformat.1.3.html#_format_pvrtc1_4bpp

Current values are (bc[7]&1) which is bit 56.

bimg/src/image.cpp

Line 3044 in 82738cd

const bool punchthrough = !!(bc[7] & 1);

bimg/src/image.cpp

Line 3152 in 82738cd

const bool punchthrough = !!(bc[7] & 1);

Other problem is that with punchthrough:
"If punch-through mode is selected, and the modulation bits for a given pixel have a value of 0b10, the alpha value of the resulting color is 0x00. This is irrespective of the presence or values of any alpha channel in the input colors."

image decoder uses s_pvrtcWeights table for weights. Outputting 0 can be implemented by settings alpha weights to 0 when punchtrough is on and modulation is 0b10.

bimg/src/image.cpp

Line 2969 in 82738cd

{ 4, 4, 4, 4 },

Alpha weights are the 3. and 4. values.
So this should look like
{ 4, 4, 0, 0 },

bkaradzic/bgfx#792