library: sort out vertex2d UV API confusion

src/library/doc.texi

@@ -1939,6 +1939,18 @@ associated image in the batch's texture atlas, in pixel coordinates.
 
 Given an index of a vertex in a batch, returns the width and height of
 its associated image in the batch's texture atlas, in pixel coordinates.
+The game engine has a strangely complex system for these values, which
+is described in the next paragraph. This is the type of thing which the
+Bolt API would normally make simple using abstraction, but this can't
+easily be abstracted, so the values are left exactly as-is. Plugins will
+need to account for everything in the following paragraph if they intend
+to work with UV coordinates.
+
+If the U value is less than -60000, then the texture info for this
+vertex should be ignored and RGBA values of 1.0 should be used.
+Otherwise, if a UV value is negative, it should be negated (i.e. made
+positive) and then clamped to the edges of the sub-image. If a UV value
+is positive, it should wrap within the sub-image.
 
 @node batch2d-vertexuv
 @subsection vertexuv
@@ -1948,12 +1960,10 @@ Given an index of a vertex in a batch, returns the vertex's associated
 0.0 - 1.0.
 
 Unlike with 3D render events, these UVs are relative to the entire
-texture atlas, not to the sub-image. However they may fall outside the
-bounds of the sub-image, in which case they're usually expected to wrap
-around within it.
-
-The values will be floating-point numbers in the range 0.0 - 1.0. They
-are relative to image in the texture atlas.
+texture atlas, not to the sub-image. There are a few other things to be
+aware of when working with Render2D UVs. @ref{batch2d-vertexatlaswh,,
+vertexatlaswh}. Plugins will need to negotiate all of this correctly if
+they intend to work with UVs.
 
 @node batch2d-vertexcolour
 @subsection vertexcolour

src/library/gl.c

@@ -1689,12 +1689,15 @@ void _bolt_gl_onViewport(GLint x, GLint y, GLsizei width, GLsizei height) {
 static void _bolt_gl_plugin_drawelements_vertex2d_xy(size_t index, void* userdata, int32_t* out) {
     struct GLPluginDrawElementsVertex2DUserData* data = userdata;
     if (_bolt_get_attr_binding_int(data->c, data->position, data->indices[index], 2, out)) {
-        out[1] = (int32_t)data->screen_height - (out[1] + 1);
+        // this line would seem like it introduces an off-by-one error, but the same error actually exists in the game engine
+        // causing the UI to be drawn one pixel higher than the top of the screen, so we're just accounting for that here.
+        out[1] = (int32_t)data->screen_height - out[1];
     } else {
         float pos[2];
         _bolt_get_attr_binding(data->c, data->position, data->indices[index], 2, pos);
         out[0] = (int32_t)roundf(pos[0]);
-        out[1] = (int32_t)data->screen_height - ((int32_t)roundf(pos[1]) + 1);
+        // as above
+        out[1] = (int32_t)data->screen_height - (int32_t)roundf(pos[1]);
     }
 }
 
@@ -1710,9 +1713,8 @@ static void _bolt_gl_plugin_drawelements_vertex2d_atlas_wh(size_t index, void* u
     struct GLPluginDrawElementsVertex2DUserData* data = userdata;
     float wh[2];
     _bolt_get_attr_binding(data->c, data->atlas_size, data->indices[index], 2, wh);
-    // these are negative for some reason
-    out[0] = -(int32_t)roundf(wh[0] * data->atlas->width);
-    out[1] = -(int32_t)roundf(wh[1] * data->atlas->height);
+    out[0] = (int32_t)roundf(wh[0] * data->atlas->width);
+    out[1] = (int32_t)roundf(wh[1] * data->atlas->height);
 }
 
 static void _bolt_gl_plugin_drawelements_vertex2d_uv(size_t index, void* userdata, double* out) {