Draw display math numbering outside of inset

This is the first (easiest) step in fixing bugs 10668 and 11333.

The numbering is now drawn outside of the insets, which solves the
alignment problems and make editing easier.

What does not work yet:

- long labels will overwrite equations. To fix this, we need to
  implement the same algorithm as LaTeX and put labels on their own
  row when required.

- previews may need to be adapted similarly to fit the whole screen width

src/MetricsInfo.cpp

@@ -132,7 +132,8 @@ MetricsInfo::MetricsInfo(BufferView * bv, FontInfo font, int textwidth,
 
 PainterInfo::PainterInfo(BufferView * bv, lyx::frontend::Painter & painter)
 	: pain(painter), ltr_pos(false), change_(), selected(false),
-	do_spellcheck(true), full_repaint(true), background_color(Color_background)
+	  do_spellcheck(true), full_repaint(true), background_color(Color_background),
+	  leftx(0), rightx(0)
 {
 	base.bv = bv;
 }

src/MetricsInfo.h

@@ -142,6 +142,8 @@ public:
 	bool full_repaint;
 	/// Current background color
 	ColorCode background_color;
+	/// Useful for drawing display math numbering
+	int leftx, rightx;
 };
 
 class TextMetricsInfo {};

src/TextMetrics.cpp

@@ -48,6 +48,8 @@
 #include "support/convert.h"
 #include "support/debug.h"
 #include "support/lassert.h"
+#include "support/lyxlib.h"
+#include "support/RefChanger.h"
 
 #include <stdlib.h>
 #include <cmath>
@@ -1818,6 +1820,9 @@ void TextMetrics::drawParagraph(PainterInfo & pi, pit_type const pit, int const
 	if (pm.rows().empty())
 		return;
 	size_t const nrows = pm.rows().size();
+	// Remember left and right margin for drawing math numbers
+	Changer changeleft = make_change(pi.leftx, x + leftMargin(pit));
+	Changer changeright = make_change(pi.rightx, x + width() - rightMargin(pit));
 
 	// Use fast lane in nodraw stage.
 	if (pi.pain.isNull()) {
@@ -1864,6 +1869,9 @@ void TextMetrics::drawParagraph(PainterInfo & pi, pit_type const pit, int const
 		}
 	}
 
+	if (text_->isRTL(pit))
+		swap(pi.leftx, pi.rightx);
+
 	for (size_t i = 0; i != nrows; ++i) {
 
 		Row const & row = pm.rows()[i];

src/mathed/InsetMathHull.cpp

@@ -560,17 +560,6 @@ void InsetMathHull::metrics(MetricsInfo & mi, Dimension & dim) const
 		dim.des += display_margin;
 	}
 
-	if (numberedType()) {
-		Changer dummy = mi.base.changeFontSet("mathrm");
-		int l = 0;
-		for (row_type row = 0; row < nrows(); ++row)
-			l = max(l, mathed_string_width(mi.base.font, nicelabel(row)));
-
-		if (l)
-			// Value was hardcoded to 30 pixels
-			dim.wid += mi.base.bv->zoomedPixels(30) + l;
-	}
-
 	// reserve some space for marker.
 	dim.wid += 2;
 }
@@ -647,45 +636,33 @@ void InsetMathHull::draw(PainterInfo & pi, int x, int y) const
 		return;
 	}
 
+	// First draw the numbers
 	ColorCode color = pi.selected && lyxrc.use_system_colors
 				? Color_selectiontext : standardColor();
 	bool const really_change_color = pi.base.font.color() == Color_none;
 	Changer dummy0 = really_change_color ? pi.base.font.changeColor(color)
 		: Changer();
-	Changer dummy1 = pi.base.changeFontSet(standardFont());
-	Changer dummy2 = pi.base.font.changeStyle(display() ? DISPLAY_STYLE
-	                                                    : TEXT_STYLE);
-
-	int xmath = x;
-	BufferParams::MathNumber const math_number = buffer().params().getMathNumber();
-	if (numberedType() && math_number == BufferParams::LEFT) {
-		Changer dummy = pi.base.changeFontSet("mathrm");
-		int l = 0;
-		for (row_type row = 0; row < nrows(); ++row)
-			l = max(l, mathed_string_width(pi.base.font, nicelabel(row)));
-
-		if (l)
-			// Value was hardcoded to 30 pixels
-			xmath += pi.base.bv->zoomedPixels(30) + l;
-	}
-
-	InsetMathGrid::draw(pi, xmath + 1, y);
-	drawMarkers(pi, x, y);
-
-	if (numberedType()) {
-		Changer dummy = pi.base.changeFontSet("mathrm");
+	if (pi.full_repaint && numberedType()) {
+		BufferParams::MathNumber const math_number = buffer().params().getMathNumber();
 		for (row_type row = 0; row < nrows(); ++row) {
 			int const yy = y + rowinfo(row).offset;
 			docstring const nl = nicelabel(row);
-			if (math_number == BufferParams::LEFT)
-				pi.draw(x, yy, nl);
-			else {
-				int l = mathed_string_width(pi.base.font, nl);
-				pi.draw(x + dim.wid - l, yy, nl);
+			if (math_number == BufferParams::LEFT) {
+				pi.draw(pi.leftx, yy, nl);
+			} else {
+				int const l = mathed_string_width(pi.base.font, nl);
+				pi.draw(pi.rightx - l, yy, nl);
 			}
 		}
 	}
 
+	// Then the equations
+	Changer dummy1 = pi.base.changeFontSet(standardFont());
+	Changer dummy2 = pi.base.font.changeStyle(display() ? DISPLAY_STYLE
+	                                                    : TEXT_STYLE);
+	InsetMathGrid::draw(pi, x + 1, y);
+	drawMarkers(pi, x, y);
+
 	// drawing change line
 	if (canPaintChange(*bv))
 		pi.change_.paintCue(pi, x + 1, y + 1 - dim.asc,