fix: point-filter window for even DPI factors

We emulate DPI scaling by drawing to a smaller render target and scale
it up with bilinear filtering to the desired DPI scale factor configured
in the OS.

Windows seems to do crisper point filtering (nearest neighbour) for
scale factors that are multiples of 100%.

This commit alters our behaviour to select the filter mode based on the
DPI scale factor, opting for point filtering for non-fractional scaling.

Author: zao

engine/render/r_main.cpp

@@ -1134,6 +1134,7 @@ void r_renderer_c::BeginFrame()
 		auto& vid = sys->video->vid;
 		int wNew = VirtualScreenWidth();
 		int hNew = VirtualScreenHeight();
+		bool const wantIntegerScaling = fmodf(vid.dpiScale, 1.0f) < 0.0005f;
 		for (int i = 0; i < 2; ++i) {
 			auto& rtt = rttMain[i];
 			if (rtt.width != wNew || rtt.height != hNew) {
@@ -1144,8 +1145,9 @@ void r_renderer_c::BeginFrame()
 				glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, wNew, hNew, 0, GL_RGB, GL_UNSIGNED_BYTE, nullptr);
 				glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
 				glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
-				glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
-				glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
+				GLint const filterMode = wantIntegerScaling ? GL_NEAREST : GL_LINEAR;
+				glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, filterMode);
+				glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, filterMode);
 
 				rtt.width = wNew;
 				rtt.height = hNew;