Blentran: implement linedef actions for all blendmode strengths

The new blending mode features for midtextures and FOFs from !1534 (closed) are excellent, but only allow additive/subtractive/reverse subtractive midtextures at 100% strength. Instead of only using actions 910 to 913 for additive, subtractive, reverse subtractive and modulate respectively, this branch changes it to the following:

• 910 to 919 are used for 100% additive to 10% additive.
• 920 to 929 are used for 100% subtractive to 10% subtractive.
• 930 to 939 are used for 100% reverse subtractive to 10% reverse subtractive.
• 940 is used for modulate.

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• a4042cab - Use blend mode constants instead of hardcoded action numbers.

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• b84199fa - Simplify some code at toaster's suggestion.

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• 109c60f7 - Do the same for OpenGL, and also make modulate actually work in it.

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merged

mentioned in commit d9ecb800