Don't parse multicolumn if the grid does not support it

Fixes #10466 and visual/compilation bugs with InsetMathSplit, InsetMathDiagram,
InsetMathHull, InsetMathSubstack and InsetMathXYMatrix.

src/mathed/InsetMathAMSArray.h

@@ -58,6 +58,8 @@ public:
 	int leftMargin() const { return 6; } //override
 	///
 	int rightMargin() const { return 8; } //override
+	///
+	bool handlesMulticolumn() const { return true; } //override
 
 private:
 	virtual Inset * clone() const;

src/mathed/InsetMathArray.h

@@ -53,6 +53,8 @@ public:
 	int leftMargin() const { return 4; } //override
 	///
 	int rightMargin() const { return 2; } //override
+	///
+	bool handlesMulticolumn() const { return true; } //override
 
 private:
 	virtual Inset * clone() const;

src/mathed/InsetMathCases.h

@@ -56,6 +56,8 @@ public:
 	int leftMargin() const { return 8; } //override
 	///
 	int rightMargin() const { return 0; } //override
+	/// see e.g. https://tex.stackexchange.com/a/133283/87201
+	bool handlesMulticolumn() const { return true; } //override
 
 private:
 	virtual Inset * clone() const;

src/mathed/InsetMathGrid.h

@@ -211,6 +211,8 @@ public:
 	virtual int vlinesep() const;
 	///
 	virtual int border() const;
+	///
+	virtual bool handlesMulticolumn() const { return false; }
 
 	///
 	void write(WriteStream & os) const;

src/mathed/InsetMathHull.cpp

@@ -15,7 +15,6 @@
 #include "InsetMathChar.h"
 #include "InsetMathColor.h"
 #include "InsetMathFrac.h"
-#include "InsetMathGrid.h"
 #include "InsetMathNest.h"
 #include "InsetMathScript.h"
 #include "MathExtern.h"

src/mathed/InsetMathMatrix.h

@@ -47,6 +47,8 @@ public:
 	void octave(OctaveStream &) const;
 	///
 	InsetCode lyxCode() const { return MATH_MATRIX_CODE; }
+	///
+	bool handlesMulticolumn() const { return true; } //override
 
 private:
 	virtual Inset * clone() const;

src/mathed/InsetMathSplit.cpp

@@ -48,11 +48,6 @@ Inset * InsetMathSplit::clone() const
 }
 
 
-// FIXME: InsetMathGrid should be changed to let the real column alignment be
-// given by a virtual method like displayColAlign, because the values produced
-// by defaultColAlign can be invalidated by lfuns such as add-column. I suspect
-// that for the moment the values produced by defaultColAlign are not used,
-// notably because alignment is not implemented in the LyXHTML output.
 char InsetMathSplit::defaultColAlign(col_type col)
 {
 	if (name_ == "gathered")

src/mathed/InsetMathTabular.h

@@ -49,6 +49,8 @@ public:
 	int leftMargin() const { return 4; } //override
 	///
 	int rightMargin() const { return 2; } //override
+	///
+	bool handlesMulticolumn() const { return true; } //override
 
 private:
 	Inset * clone() const;

src/mathed/MathParser.cpp

@@ -1363,7 +1363,7 @@ bool Parser::parse1(InsetMathGrid & grid, unsigned flags,
 			}
 		}
 
-		else if (t.cs() == "multicolumn") {
+		else if (t.cs() == "multicolumn" && grid.handlesMulticolumn()) {
 			// if the columns are specified numerically,
 			// extract column count and insert dummy cells,
 			// otherwise parse it as an user macro