tr_shader: do not detect heightmap in normalmap alpha channel, unless asked to, fix #375

[illwieckz]

With this PR,

• normalMap material keyword loads a file and assume it's a normal map without height map in alpha channel (even if the engine believes there is an alpha channel because of a format being wrongly guessed on missing GL extension, see Bad normal map channel detection, seems to be related to missing ARB_texture_compression_rgtc #375).
• normalHeightMap material keyword loads a file it's a normal map with an height map in alpha channel.

This code also introduces an internal ParseNormalMapDetectHeightMap function that attempts to detect an alpha channel and declare it's an heightmap. There is no material keyword for that and it must not be implemented as a keyword.

This internal function is only purposed to be used by compatibility code like the Darkplaces compatibility mode. Darkplaces-based games being known to not use formats that would break on detection, it's seems to be safe to use this detection function for their assets.

[illwieckz]

@slipher do you have any remark on it? I would appreciate some remark on pointer wizardry to keep an unmodified copy of **text if there is a better implementation possible.

[slipher]

I suggest as follows. Notes:

• DoDebugCode is used to execute the code only if the log level is debug.
• char buffer[1024] = ""; requires the entire buffer to be zeroed.

	const char* initialText = *text;

	ParseNormalMap( stage, text, bundleIndex );

	/* Tell renderer to enable relief mapping since an heightmap is found
	also tell renderer to not abuse normalmap alpha channel because it's an heightmap.
	See https://github.com/DaemonEngine/Daemon/issues/183#issuecomment-473691252 */
	if ( stage->bundle[ bundleIndex ].image[ 0 ]->bits & IF_NORMALMAP
		&& stage->bundle[ bundleIndex ].image[ 0 ]->bits & IF_ALPHA )
	{
		Log::defaultLogger.DoDebugCode([&] {
			char buffer[ 1024 ];
			buffer[ 0 ] = '\0'
			bool parsed = ParseMap( &initialText, buffer, sizeof( buffer ) );
			ASSERT( parsed );
			Log::Debug("Found heightmap embedded in normalmap '%s'", buffer);
		});

		stage->isHeightMapInNormalMap = true;
	}

[illwieckz]

@slipher, I rewrote the things accordingly. I added a UNUSED keyword to silent the compiler complaining about parsed being unused on non-debug code. Or I may hack something like that instead:

			#ifdef DEBUG
			bool parsed = 
			#endif
			ParseMap( &initialText, buffer, sizeof( buffer ) );

[slipher]

There is already a facility for marking an unused variable, Q_UNUSED. It's used like Q_UNUSED(variable);

Alternatively if(!parsed) ASSERT(false); would work.

[illwieckz]

Hmm, since all those tricks are there to move the ParseMap call out of ASSERT, then we can just avoid a bool and do:

			if ( !ParseMap( &initialText, buffer, sizeof( buffer ) ) )
			{
				ASSERT( false );
			}