Fix build against ImGui >= 1.92.8 (ImLerp cannot take ImOffsetRect)

[tonzsm]

Body

Closes #21.

### Problem

`imgui_toggle` does not compile against any ImGui >= 1.92.8.

1.92.8 redefined the generic lerp in `imgui_internal.h`:

```cpp
// <= 1.92.2b
template<typename T> T ImLerp(T a, T b, float t) { return (T)(a + (b - a) * t); }

// >= 1.92.8  (ocornut/imgui@f2f843c)
template<typename T> T ImLerp(T a, T b, float t) { return (T)((float)a + (float)(b - a) * t); }

The new form needs a T -> float conversion. ImOffsetRect only converts the other way (via its
ImOffsetRect(float all) constructor), so instantiating the template fails — and
ImGuiToggleRenderer::UpdateState() lerps two of them:

// imgui_toggle_renderer.cpp:409-410
_state.KnobInset  = ImLerp(_config.Off.KnobInset,  _config.On.KnobInset,  _animationPercent);
_state.KnobOffset = ImLerp(_config.Off.KnobOffset, _config.On.KnobOffset, _animationPercent);

giving:

imgui_internal.h(524): error C2440: 'type cast': cannot convert from 'ImOffsetRect' to 'float'
        with [ T=ImOffsetRect ]
imgui_toggle_renderer.cpp(409): note: see reference to function template instantiation
        'T ImLerp<ImOffsetRect>(T,T,float)' being compiled

Pinning an older ImGui is not a way out: #20 ("Fix for ImGui 1.92.8 AddRect arg swap") already requires

= 1.92.8, so going older just trades this break for the AddRect one. Right now there is no ImGui
version that builds.

Fix

This is @nitz's own fix from #21, unchanged — an exact-match ImLerp overload for ImOffsetRect at
the end of imgui_offset_rect.h, guarded on IMGUI_VERSION_NUM >= 19274:

#if IMGUI_VERSION_NUM >= 19274
static inline ImOffsetRect ImLerp(ImOffsetRect a, ImOffsetRect b, float t) { return (a + (b - a) * t); }
#endif

An exact match beats ImGui's template during overload resolution, so the two call sites above pick it up
with no other change. The result is identical to what the old template produced: t converts to
ImOffsetRect(t,t,t,t) and the existing operator* multiplies component-wise.

I added only a comment explaining why it exists and IM_MSVC_RUNTIME_CHECKS_OFF/RESTORE to match the
neighbouring operators.

#21 was closed as completed, but the fix only ever existed as a comment — so anyone cloning main still
hits this. This commits it.

Verification

• Builds against ImGui 1.92.8 (IMGUI_VERSION_NUM 19280), MSVC, /std:c++17 /W4, no warnings.
• Toggle animation unchanged (knob inset/offset still interpolate).
• No effect on ImGui < 19274 (guarded out).