Optimize rendering runs of spaces when only the foreground changes

cmatrix is somewhat of a pathological case for our infrastructure: it
prints out a bunch of green and white characters and then updates them a
million times a second.

It also maintains a column of space between every green character. When
it prints this column, it prints it in "default" or "white". This ends
up making runs of text that look like this:

(def: G=green B=bright white W=white *=matrix char  =space)

G W G W G W G W G W G W G W G W
G W G W G W G W G W G W G W G W
G W G W G W G W G W G W G W G W
G W G W G W G W G W G W G W G W
G W G W G W G W G W G W G W G W
G W G W G W G W G W G W G W G W
G W G W G W G W G W G W G W G W
G W G W G W G W G W G W G W G W

As characters trickle in:

G*W G*W G*W G*W G*W G*W G*W B*W
G*W G*W G*W G*W G*W G*W G*W G W
G*W G*W G*W B*W G*W G*W G*W G W
G*W B*W G*W G W G*W G*W G*W G*W
G*W G W G*W G W G*W B*W G*W G*W
B*W G W G*W G W G*W G W B*W G*W
G W G W G*W G W G*W G W G W B*W
G W G W B*W G W G*W G W G W G W

Every one of those color transitions causes us to break up the run of
text and start rendering it again. This impacts GDI, Direct2D *and*
ConPTY.

The problem is, printing a space doesn't **use** the foreground color!

This commit introduces an optimization. When we're about to break a text
cluster becuase its attributes changed, we make sure that it's not just
filled with spaces and differs only in the foreground parameter.

This lets us optimize both the rendering _and_ the PTY output to look
like this:

G*   *   *   *   *   *   *  B*
G*   *   *   *   *   *   *
G*   *   *  B*   *   *   *
G*  B*   *       *   *   *   *
G*       *       *  B*   *   *
B*       *       *      B*   *
G        *       *          B*
G       B*       *

Text will be printed at best line-by-line and at worst only when the
visible properties of the screen actually change.

This speeds up cmatrix remarkably.

src/buffer/out/TextAttribute.hpp

@@ -163,6 +163,14 @@ class TextAttribute final
         return _foreground.IsRgb() || _background.IsRgb();
     }
 
+    // In certain scenarios, we don't care about specifically the foreground color.
+    constexpr bool EqualsExceptForeground(const TextAttribute& other) const noexcept
+    {
+        return (_wAttrLegacy & META_ATTRS) == (other._wAttrLegacy & META_ATTRS) &&
+               _background == other._background &&
+               _extendedAttrs == other._extendedAttrs;
+    }
+
 private:
     COLORREF _GetRgbForeground(std::basic_string_view<COLORREF> colorTable,
                                COLORREF defaultColor) const noexcept;

src/renderer/base/renderer.cpp

@@ -610,6 +610,12 @@ void Renderer::_PaintBufferOutput(_In_ IRenderEngine* const pEngine)
     }
 }
 
+static bool _IsAllSpaces(const std::wstring_view v)
+{
+    // first non-space char is not found (is npos)
+    return v.find_first_not_of(L" ") == decltype(v)::npos;
+}
+
 void Renderer::_PaintBufferOutputHelper(_In_ IRenderEngine* const pEngine,
                                         TextBufferCellIterator it,
                                         const COORD target,
@@ -661,8 +667,14 @@ void Renderer::_PaintBufferOutputHelper(_In_ IRenderEngine* const pEngine,
             {
                 if (color != it->TextAttr())
                 {
-                    color = it->TextAttr();
-                    break;
+                    auto newAttr{ it->TextAttr() };
+                    // foreground doesn't matter for runs of spaces (!)
+                    // if we trick it . . . we call Paint far fewer times for cmatrix
+                    if (!_IsAllSpaces(it->Chars()) || !newAttr.EqualsExceptForeground(color))
+                    {
+                        color = newAttr;
+                        break; // vend this run
+                    }
                 }
 
                 // Walk through the text data and turn it into rendering clusters.