fix blended textures for dx, opengl, and metal

[Archez]

This PR adds some fixes around the masked/blended texture shaders

The blended textures shader effect was not fully implemented on DirectX. DirectX has 3 code paths:

1. Tri-point filtering is off
2. Tri-point filtering is on with linear filtering enabled for the texture
3. Tri-point filtering is on but linear filtering is disabled for the texture

This third path was missing the logic for masked/blended texture effects. I have copied the patterns from code path 1 to here.

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OpenGL, unlike the other renders, seems to require a dedicated sampler for each texture. We were only setting a sampler for the main textures (index 0 and 1), but not for the blended textures (index 4 and 5). This meant that any parameters needed for the texture were not applied to the blended texture in OpenGL like GL_MIRRORED_REPEAT to mirror a texture outside the texture coordinates. This PR checks if the texture has a blended texture and then applies the same sampler parameters to the blended texture index so OpenGL renders the texture as expected.

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I also noticed a buffer size mismatch in the metal shader where it was updated to 8192 in the parent caller that passes the buffer through, but the function signature was still at the old size.