Fix MultiTexture support

[0x1F9F1]

Although AGI appears to support multi-texturing, support has been removed in a few places:
NoMultiTexture is always set to 1.
agiCurState.MaxTextures is always set to 1.

agiD3DPipeline::BeginGfx:

// test1560.exe

NoMultiTexture = 1;
if (hwcaps.dwDevCaps & 0x4000) {
    agiDisplayf("Separate texture memories (probably a voodoo!)");
    NoMultiTexture = 1;
}
agiCurState.State.MaxTextures = 1;
SeparateTextureMemories = 0;

// am.exe

NoMultiTexture = 1;
if (hwcaps.dwDevCaps & 0x4000) {
    agiDisplayf("Separate texture memories (probably a voodoo!)");
    SeparateTextureMemories = 1;
    NoMultiTexture = 1;
}
if (NoMultiTexture)
    agiRendState::SetMaxTextures(&agiCurState, 1);
else
    agiRendState::SetMaxTextures(&agiCurState, hwcaps.wMaxSimultaneousTextures);
if (agiRendState::GetMaxTextures((char*)&agiCurState) <= 1) {
    SeparateTextureMemories = 0;
} else {
    agiDisplayf("Multitexturing is supported!");
    v41 = v49->D3DDevice->lpVtbl->SetTextureStageState(v49->D3DDevice, 1, D3DTSS_TEXCOORDINDEX, 1);
    __DDTry(
        v41,
        (int)"D3DDevice->SetTextureStageState(1,D3DTSS_TEXCOORDINDEX,1)",
        (int)"C:\\mm\\src\\agid3d\\d3dpipe.c",
        583);
}

agiD3DTexDef::BeginGfx:

// am.exe

multi_texture = 0;

// ...
if (multi_texture
    && agiRendState::GetMaxTextures(&agiCurState) > 1
    && !strncmp(this->Params.Name, "SHADMAP", 7u)) {
    Displayf("Fixing shadow map '%s' for multitexturing...", this->Params.Name);
    surface = (uint16_t*)this->Surface->lpSurface;
    surface_bytes = 0;
    v19 = this->Surface->dwWidth;
    for (i = this->Surface->dwHeight; v19 && i; i >>= 1) {
        surface_bytes += i * v19;
        v19 >>= 1;
    }
    while (surface_bytes--) {
        *surface = ~(((signed int)*surface >> 12) | ((signed int)*surface >> 8) | ((signed int)*surface >> 4));
        ++surface;
    }
}
// ...