Use one UI drop shadow for text and atlas images, fix Windows build

Text and the SVG button images are mixed on screen, so they now share
UIDropShadowParams (3 UI units of blur, 0.66 intensity, no offset, the
atlas's existing look) instead of the text having its own.

draw_text.cpp included <windows.h> directly, whose min/max macros broke
the std::max calls added here. Include CommonWindows.h instead.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Common/Render/DropShadow.cpp

@@ -20,6 +20,17 @@
 
 #include "Common/Render/DropShadow.h"
 
+// In UI units.
+static constexpr float UI_SHADOW_RADIUS = 3.0f;
+static constexpr float UI_SHADOW_INTENSITY = 0.66f;
+
+DropShadowParams UIDropShadowParams(float pixelsPerUnit) {
+	DropShadowParams params;
+	params.radius = std::max(1, (int)(UI_SHADOW_RADIUS * pixelsPerUnit));
+	params.intensity = UI_SHADOW_INTENSITY;
+	return params;
+}
+
 void CompositeDropShadow(const u32 *src, int w, int h, u32 *dst, int dstStride, const DropShadowParams &params) {
 	const int radius = std::max(1, params.radius);
 	const int pad = params.Padding();