Add KTX1 texture container support with spec-compliant reader

[Copilot]

Adds KTX1 (Khronos Texture) read support to CBitmap, enabling .ktx files with compressed and uncompressed pixel data.

New file: bitmap_ktx.cpp

• Compressed formats: DXTC1, DXTC1Alpha, DXTC3, DXTC5 (ETC1 recognized but unsupported by bitmap loader)
• Uncompressed formats: RGBA, RGB→RGBA expansion, Luminance, Alpha, LuminanceAlpha
• Mipmap chain loading with mipMapSkip support matching DDS behavior

Spec compliance details

• Endianness: Uses raw serialBuffer() reads instead of IStream::serial() to avoid conflicting with KTX's own endianness mechanism — serial() does platform byte-swapping on big-endian systems which would double-swap when combined with KTX's endianness field
• Row alignment: Accounts for GL_UNPACK_ALIGNMENT=4 row padding per spec — rows are padded to 4-byte boundaries, stripped during load
• 1D textures: pixelHeight == 0 treated as height 1 per spec
• Endianness validation: Rejects files where the endianness field is neither 0x04030201 nor 0x01020304

// Raw read avoids IStream::serial() byte-swapping on big-endian platforms
static void ktxReadRaw32(NLMISC::IStream &f, uint32 &val)
{
    f.serialBuffer((uint8 *)&val, 4);
}

Integration points

• bitmap.h: KTX_HEADER constant, readKTX() declaration
• bitmap.cpp: KTX dispatch in load() and loadSize()
• CMake: Picked up automatically by existing bitmap_*.cpp glob

Original prompt

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This section details on the original issue you should resolve

<issue_title>Support .ktx texture container</issue_title>
<issue_description>Currently only .dds is supported, supporting .ktx container, which supports more codecs, may help towards mobile and modern GPU support.</issue_description>

Comments on the Issue (you are @copilot in this section)

@kaetemi KTX1 https://www.khronos.org/opengles/sdk/tools/KTX/file_format_spec/ KTX2 http://github.khronos.org/KTX-Specification/

• Fixes Support .ktx texture container #610

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