GPU: Round horizontal display range values down to clockdiv multiple

GP1(06h) X1 and X2 are persistent across clockdiv changes, but the GPU
actively uses rounded values for display output. This behavior is
modeled by storing the rounded values in horizontal_display_start and
horizontal_display_end. Verified with hardware test.

src/core/gpu.cpp

@@ -481,8 +481,10 @@ void GPU::UpdateCRTCConfig()
 
   const u8 horizontal_resolution_index = m_GPUSTAT.horizontal_resolution_1 | (m_GPUSTAT.horizontal_resolution_2 << 2);
   cs.dot_clock_divider = dot_clock_dividers[horizontal_resolution_index];
-  cs.horizontal_display_start = std::min<u16>(cs.regs.X1, cs.horizontal_total);
-  cs.horizontal_display_end = std::min<u16>(cs.regs.X2, cs.horizontal_total);
+  cs.horizontal_display_start =
+    (std::min<u16>(cs.regs.X1, cs.horizontal_total) / cs.dot_clock_divider) * cs.dot_clock_divider;
+  cs.horizontal_display_end =
+    (std::min<u16>(cs.regs.X2, cs.horizontal_total) / cs.dot_clock_divider) * cs.dot_clock_divider;
   cs.vertical_display_start = std::min<u16>(cs.regs.Y1, cs.vertical_total);
   cs.vertical_display_end = std::min<u16>(cs.regs.Y2, cs.vertical_total);
 
@@ -524,8 +526,10 @@ void GPU::UpdateCRTCDisplayParameters()
 
   const u16 horizontal_total = m_GPUSTAT.pal_mode ? PAL_TICKS_PER_LINE : NTSC_TICKS_PER_LINE;
   const u16 vertical_total = m_GPUSTAT.pal_mode ? PAL_TOTAL_LINES : NTSC_TOTAL_LINES;
-  const u16 horizontal_display_start = std::min<u16>(cs.regs.X1, horizontal_total);
-  const u16 horizontal_display_end = std::min<u16>(cs.regs.X2, horizontal_total);
+  const u16 horizontal_display_start =
+    (std::min<u16>(cs.regs.X1, horizontal_total) / cs.dot_clock_divider) * cs.dot_clock_divider;
+  const u16 horizontal_display_end =
+    (std::min<u16>(cs.regs.X2, horizontal_total) / cs.dot_clock_divider) * cs.dot_clock_divider;
   const u16 vertical_display_start = std::min<u16>(cs.regs.Y1, vertical_total);
   const u16 vertical_display_end = std::min<u16>(cs.regs.Y2, vertical_total);