UltraHDRLoader: Fix incorrect alpha channel values.

[mrdoob]

Related issue: #28825 #32380

Description

Fixed incorrect alpha channel initialization values in the HDR buffer.

For HalfFloatType, the fill value was 23544 which represents 255 in half-float encoding. The correct value for alpha 1.0 is 15360 (0x3C00).

For FloatType, the fill value was 255 instead of 1.0.

/fyi @mjurczyk

[WestLangley]

For HalfFloatType, the fill value was 23544 which represents ~127.75 in half-float encoding.

three.js utils says it represents 255.

[mrdoob]

You're right, I made an error in my half-float conversion. Let me verify:

23544 = 0x5BF8
Binary: 0 10110 1111111000
      sign=0, exponent=22, mantissa=1016

Value = 2^(22-15) * (1 + 1016/1024) = 2^7 * 1.9921875 = 255

So 23544 represents 255, not ~127.75. My apologies for the incorrect conversion! The bug is still the same though - alpha should be 1.0 (which is 15360 in half-float), not 255 (which is 23544). The fix is correct.